diff --git a/dom/base/FormData.cpp b/dom/base/FormData.cpp
index de4d71a676..d47a970fc8 100644
--- a/dom/base/FormData.cpp
+++ b/dom/base/FormData.cpp
@@ -398,7 +398,7 @@ FormData::Constructor(const GlobalObject& aGlobal,
 
 NS_IMETHODIMP
 FormData::GetSendInfo(nsIInputStream** aBody, uint64_t* aContentLength,
-                      nsACString& aContentType, nsACString& aCharset)
+                      nsACString& aContentTypeWithCharset, nsACString& aCharset)
 {
   FSMultipartFormData fs(NS_LITERAL_CSTRING("UTF-8"), nullptr);
 
@@ -419,7 +419,7 @@ FormData::GetSendInfo(nsIInputStream** aBody, uint64_t* aContentLength,
     }
   }
 
-  fs.GetContentType(aContentType);
+  fs.GetContentType(aContentTypeWithCharset);
   aCharset.Truncate();
   *aContentLength = 0;
   NS_ADDREF(*aBody = fs.GetSubmissionBody(aContentLength));
diff --git a/dom/base/Navigator.cpp b/dom/base/Navigator.cpp
index ef8a453742..894e0a162e 100644
--- a/dom/base/Navigator.cpp
+++ b/dom/base/Navigator.cpp
@@ -11,7 +11,9 @@
 #include "nsPluginArray.h"
 #include "nsMimeTypeArray.h"
 #include "mozilla/MemoryReporting.h"
+#include "mozilla/dom/BodyExtractor.h"
 #include "mozilla/dom/DesktopNotification.h"
+#include "mozilla/dom/FetchBinding.h"
 #include "mozilla/dom/File.h"
 #include "nsGeolocation.h"
 #include "nsIClassOfService.h"
@@ -857,8 +859,52 @@ BeaconStreamListener::OnDataAvailable(nsIRequest *aRequest,
 
 bool
 Navigator::SendBeacon(const nsAString& aUrl,
-                      const Nullable<ArrayBufferViewOrBlobOrStringOrFormData>& aData,
+                      const Nullable<ArrayBufferOrArrayBufferViewOrBlobOrFormDataOrUSVStringOrURLSearchParams>& aData,
                       ErrorResult& aRv)
+{
+  if (aData.IsNull()) {
+    return SendBeaconInternal(aUrl, nullptr, /* isBlob */ false, aRv);
+  }
+
+  if (aData.Value().IsArrayBuffer()) {
+    BodyExtractor<const ArrayBuffer> body(&aData.Value().GetAsArrayBuffer());
+    return SendBeaconInternal(aUrl, &body, /* isBlob*/ false, aRv);
+  }
+
+  if (aData.Value().IsArrayBufferView()) {
+    BodyExtractor<const ArrayBufferView> body(&aData.Value().GetAsArrayBufferView());
+    return SendBeaconInternal(aUrl, &body, /* isBlob*/ false, aRv);
+  }
+
+  if (aData.Value().IsBlob()) {
+    BodyExtractor<Blob> body(&aData.Value().GetAsBlob());
+    return SendBeaconInternal(aUrl, &body, /* isBlob */ true, aRv);
+  }
+
+  if (aData.Value().IsFormData()) {
+    BodyExtractor<FormData> body(&aData.Value().GetAsFormData());
+    return SendBeaconInternal(aUrl, &body, /* isBlob */ false, aRv);
+  }
+
+  if (aData.Value().IsUSVString()) {
+    BodyExtractor<const nsAString> body(&aData.Value().GetAsUSVString());
+    return SendBeaconInternal(aUrl, &body, /* isBlob */ false, aRv);
+  }
+
+  if (aData.Value().IsURLSearchParams()) {
+    BodyExtractor<URLSearchParams> body(&aData.Value().GetAsURLSearchParams());
+    return SendBeaconInternal(aUrl, &body, /* isBlob */ false, aRv);
+  }
+
+  MOZ_CRASH("Invalid data type.");
+  return false;
+}
+
+bool
+Navigator::SendBeaconInternal(const nsAString& aUrl,
+                              BodyExtractorBase* aBody,
+                              bool aIsBlob,
+                              ErrorResult& aRv)
 {
   if (!mWindow) {
     aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
@@ -900,9 +946,9 @@ Navigator::SendBeacon(const nsAString& aUrl,
     nsIChannel::LOAD_CLASSIFY_URI;
 
   // No need to use CORS for sendBeacon unless it's a BLOB
-  nsSecurityFlags securityFlags = (!aData.IsNull() && aData.Value().IsBlob())
-   ? nsILoadInfo::SEC_REQUIRE_CORS_DATA_INHERITS
-   : nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_DATA_INHERITS;
+  nsSecurityFlags securityFlags = aIsBlob
+    ? nsILoadInfo::SEC_REQUIRE_CORS_DATA_INHERITS
+    : nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_DATA_INHERITS;
   securityFlags |= nsILoadInfo::SEC_COOKIES_INCLUDE;
 
   nsCOMPtr<nsIChannel> channel;
@@ -928,67 +974,15 @@ Navigator::SendBeacon(const nsAString& aUrl,
   }
   httpChannel->SetReferrer(documentURI);
 
-  nsCString mimeType;
-  if (!aData.IsNull()) {
-    nsCOMPtr<nsIInputStream> in;
+  nsCOMPtr<nsIInputStream> in;
+  nsAutoCString contentTypeWithCharset;
+  nsAutoCString charset;
+  uint64_t length = 0;
 
-    if (aData.Value().IsString()) {
-      nsCString stringData = NS_ConvertUTF16toUTF8(aData.Value().GetAsString());
-      nsCOMPtr<nsIStringInputStream> strStream = do_CreateInstance(NS_STRINGINPUTSTREAM_CONTRACTID, &rv);
-      if (NS_FAILED(rv)) {
-        aRv.Throw(NS_ERROR_FAILURE);
-        return false;
-      }
-      rv = strStream->SetData(stringData.BeginReading(), stringData.Length());
-      if (NS_FAILED(rv)) {
-        aRv.Throw(NS_ERROR_FAILURE);
-        return false;
-      }
-      mimeType.AssignLiteral("text/plain;charset=UTF-8");
-      in = strStream;
-
-    } else if (aData.Value().IsArrayBufferView()) {
-
-      nsCOMPtr<nsIStringInputStream> strStream = do_CreateInstance(NS_STRINGINPUTSTREAM_CONTRACTID, &rv);
-      if (NS_FAILED(rv)) {
-        aRv.Throw(NS_ERROR_FAILURE);
-        return false;
-      }
-
-      const ArrayBufferView& view = aData.Value().GetAsArrayBufferView();
-      view.ComputeLengthAndData();
-      rv = strStream->SetData(reinterpret_cast<char*>(view.Data()),
-                              view.Length());
-
-      if (NS_FAILED(rv)) {
-        aRv.Throw(NS_ERROR_FAILURE);
-        return false;
-      }
-      mimeType.AssignLiteral("application/octet-stream");
-      in = strStream;
-
-    } else if (aData.Value().IsBlob()) {
-      Blob& blob = aData.Value().GetAsBlob();
-      blob.GetInternalStream(getter_AddRefs(in), aRv);
-      if (NS_WARN_IF(aRv.Failed())) {
-        return false;
-      }
-
-      nsAutoString type;
-      blob.GetType(type);
-      mimeType = NS_ConvertUTF16toUTF8(type);
-
-    } else if (aData.Value().IsFormData()) {
-      FormData& form = aData.Value().GetAsFormData();
-      uint64_t len;
-      nsAutoCString charset;
-      form.GetSendInfo(getter_AddRefs(in),
-                       &len,
-                       mimeType,
-                       charset);
-    } else {
-      MOZ_ASSERT(false, "switch statements not in sync");
-      aRv.Throw(NS_ERROR_FAILURE);
+  if (aBody) {
+    aRv = aBody->GetAsStream(getter_AddRefs(in), &length,
+                             contentTypeWithCharset, charset);
+    if (NS_WARN_IF(aRv.Failed())) {
       return false;
     }
 
@@ -997,7 +991,7 @@ Navigator::SendBeacon(const nsAString& aUrl,
       aRv.Throw(NS_ERROR_FAILURE);
       return false;
     }
-    uploadChannel->ExplicitSetUploadStream(in, mimeType, -1,
+    uploadChannel->ExplicitSetUploadStream(in, contentTypeWithCharset, length,
                                            NS_LITERAL_CSTRING("POST"),
                                            false);
   } else {
diff --git a/dom/base/Navigator.h b/dom/base/Navigator.h
index df99844ca9..16fcd4d440 100644
--- a/dom/base/Navigator.h
+++ b/dom/base/Navigator.h
@@ -30,12 +30,13 @@ class nsIURI;
 
 namespace mozilla {
 namespace dom {
+class BodyExtractorBase;
 class Geolocation;
 class systemMessageCallback;
 class MediaDevices;
 struct MediaStreamConstraints;
 class WakeLock;
-class ArrayBufferViewOrBlobOrStringOrFormData;
+class ArrayBufferOrArrayBufferViewOrBlobOrFormDataOrUSVStringOrURLSearchParams;
 class ServiceWorkerContainer;
 class DOMRequest;
 } // namespace dom
@@ -196,7 +197,7 @@ public:
 #endif // MOZ_AUDIO_CHANNEL_MANAGER
 
   bool SendBeacon(const nsAString& aUrl,
-                  const Nullable<ArrayBufferViewOrBlobOrStringOrFormData>& aData,
+                  const Nullable<ArrayBufferOrArrayBufferViewOrBlobOrFormDataOrUSVStringOrURLSearchParams>& aData,
                   ErrorResult& aRv);
 
   void MozGetUserMedia(const MediaStreamConstraints& aConstraints,
@@ -255,6 +256,11 @@ private:
   bool CheckPermission(const char* type);
   static bool CheckPermission(nsPIDOMWindowInner* aWindow, const char* aType);
 
+  bool SendBeaconInternal(const nsAString& aUrl,
+                          BodyExtractorBase* aBody,
+                          bool aIsBlob,
+                          ErrorResult& aRv);
+
   RefPtr<nsMimeTypeArray> mMimeTypes;
   RefPtr<nsPluginArray> mPlugins;
   RefPtr<Permissions> mPermissions;
diff --git a/dom/fetch/BodyExtractor.cpp b/dom/fetch/BodyExtractor.cpp
new file mode 100644
index 0000000000..889526314e
--- /dev/null
+++ b/dom/fetch/BodyExtractor.cpp
@@ -0,0 +1,236 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "BodyExtractor.h"
+#include "mozilla/dom/EncodingUtils.h"
+#include "mozilla/dom/File.h"
+#include "mozilla/dom/FormData.h"
+#include "mozilla/dom/TypedArray.h"
+#include "mozilla/dom/URLSearchParams.h"
+#include "mozilla/dom/XMLHttpRequest.h"
+#include "nsContentUtils.h"
+#include "nsIDOMDocument.h"
+#include "nsIDOMSerializer.h"
+#include "nsIInputStream.h"
+#include "nsIOutputStream.h"
+#include "nsIStorageStream.h"
+#include "nsStringStream.h"
+#include "nsIUnicodeEncoder.h"
+
+namespace mozilla {
+namespace dom {
+
+static nsresult
+GetBufferDataAsStream(const uint8_t* aData, uint32_t aDataLength,
+                      nsIInputStream** aResult, uint64_t* aContentLength,
+                      nsACString& aContentType, nsACString& aCharset)
+{
+  aContentType.SetIsVoid(true);
+  aCharset.Truncate();
+
+  *aContentLength = aDataLength;
+  const char* data = reinterpret_cast<const char*>(aData);
+
+  nsCOMPtr<nsIInputStream> stream;
+  nsresult rv = NS_NewByteInputStream(getter_AddRefs(stream), data, aDataLength,
+                                      NS_ASSIGNMENT_COPY);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  stream.forget(aResult);
+
+  return NS_OK;
+}
+
+template<> nsresult
+BodyExtractor<const ArrayBuffer>::GetAsStream(nsIInputStream** aResult,
+                                              uint64_t* aContentLength,
+                                              nsACString& aContentTypeWithCharset,
+                                              nsACString& aCharset) const
+{
+  mBody->ComputeLengthAndData();
+  return GetBufferDataAsStream(mBody->Data(), mBody->Length(),
+                               aResult, aContentLength, aContentTypeWithCharset,
+                               aCharset);
+}
+
+template<> nsresult
+BodyExtractor<const ArrayBufferView>::GetAsStream(nsIInputStream** aResult,
+                                                  uint64_t* aContentLength,
+                                                  nsACString& aContentTypeWithCharset,
+                                                  nsACString& aCharset) const
+{
+  mBody->ComputeLengthAndData();
+  return GetBufferDataAsStream(mBody->Data(), mBody->Length(),
+                               aResult, aContentLength, aContentTypeWithCharset,
+                               aCharset);
+}
+
+template<> nsresult
+BodyExtractor<nsIDocument>::GetAsStream(nsIInputStream** aResult,
+                                        uint64_t* aContentLength,
+                                        nsACString& aContentTypeWithCharset,
+                                        nsACString& aCharset) const
+{
+  nsCOMPtr<nsIDOMDocument> domdoc(do_QueryInterface(mBody));
+  NS_ENSURE_STATE(domdoc);
+  aCharset.AssignLiteral("UTF-8");
+
+  nsresult rv;
+  nsCOMPtr<nsIStorageStream> storStream;
+  rv = NS_NewStorageStream(4096, UINT32_MAX, getter_AddRefs(storStream));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIOutputStream> output;
+  rv = storStream->GetOutputStream(0, getter_AddRefs(output));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (mBody->IsHTMLDocument()) {
+    aContentTypeWithCharset.AssignLiteral("text/html;charset=UTF-8");
+
+    nsString serialized;
+    if (!nsContentUtils::SerializeNodeToMarkup(mBody, true, serialized)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    nsAutoCString utf8Serialized;
+    if (!AppendUTF16toUTF8(serialized, utf8Serialized, fallible)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    uint32_t written;
+    rv = output->Write(utf8Serialized.get(), utf8Serialized.Length(), &written);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    MOZ_ASSERT(written == utf8Serialized.Length());
+  } else {
+    aContentTypeWithCharset.AssignLiteral("application/xml;charset=UTF-8");
+
+    nsCOMPtr<nsIDOMSerializer> serializer =
+      do_CreateInstance(NS_XMLSERIALIZER_CONTRACTID, &rv);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    // Make sure to use the encoding we'll send
+    rv = serializer->SerializeToStream(domdoc, output, aCharset);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  output->Close();
+
+  uint32_t length;
+  rv = storStream->GetLength(&length);
+  NS_ENSURE_SUCCESS(rv, rv);
+  *aContentLength = length;
+
+  rv = storStream->NewInputStream(0, aResult);
+  NS_ENSURE_SUCCESS(rv, rv);
+  return NS_OK;
+}
+
+template<> nsresult
+BodyExtractor<const nsAString>::GetAsStream(nsIInputStream** aResult,
+                                            uint64_t* aContentLength,
+                                            nsACString& aContentTypeWithCharset,
+                                            nsACString& aCharset) const
+{
+  nsCOMPtr<nsIUnicodeEncoder> encoder =
+    EncodingUtils::EncoderForEncoding("UTF-8");
+  if (!encoder) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  int32_t destBufferLen;
+  nsresult rv = encoder->GetMaxLength(mBody->BeginReading(), mBody->Length(),
+                                      &destBufferLen);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  nsCString encoded;
+  if (!encoded.SetCapacity(destBufferLen, fallible)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  char* destBuffer = encoded.BeginWriting();
+  int32_t srcLen = (int32_t) mBody->Length();
+  int32_t outLen = destBufferLen;
+  rv = encoder->Convert(mBody->BeginReading(), &srcLen, destBuffer, &outLen);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  MOZ_ASSERT(outLen <= destBufferLen);
+  encoded.SetLength(outLen);
+
+  rv = NS_NewCStringInputStream(aResult, encoded);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  *aContentLength = outLen;
+  aContentTypeWithCharset.AssignLiteral("text/plain;charset=UTF-8");
+  aCharset.AssignLiteral("UTF-8");
+  return NS_OK;
+}
+
+template<> nsresult
+BodyExtractor<nsIInputStream>::GetAsStream(nsIInputStream** aResult,
+                                           uint64_t* aContentLength,
+                                           nsACString& aContentTypeWithCharset,
+                                           nsACString& aCharset) const
+{
+  aContentTypeWithCharset.AssignLiteral("text/plain");
+  aCharset.Truncate();
+
+  nsresult rv = mBody->Available(aContentLength);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIInputStream> stream(mBody);
+  stream.forget(aResult);
+  return NS_OK;
+}
+
+template<> nsresult
+BodyExtractor<Blob>::GetAsStream(nsIInputStream** aResult,
+                                 uint64_t* aContentLength,
+                                 nsACString& aContentTypeWithCharset,
+                                 nsACString& aCharset) const
+{
+  return mBody->GetSendInfo(aResult, aContentLength, aContentTypeWithCharset,
+                            aCharset);
+}
+
+template<> nsresult
+BodyExtractor<FormData>::GetAsStream(nsIInputStream** aResult,
+                                     uint64_t* aContentLength,
+                                     nsACString& aContentTypeWithCharset,
+                                     nsACString& aCharset) const
+{
+  return mBody->GetSendInfo(aResult, aContentLength, aContentTypeWithCharset,
+                            aCharset);
+}
+
+template<> nsresult
+BodyExtractor<URLSearchParams>::GetAsStream(nsIInputStream** aResult,
+                                            uint64_t* aContentLength,
+                                            nsACString& aContentTypeWithCharset,
+                                            nsACString& aCharset) const
+{
+  return mBody->GetSendInfo(aResult, aContentLength, aContentTypeWithCharset,
+                            aCharset);
+}
+
+template<> nsresult
+BodyExtractor<nsIXHRSendable>::GetAsStream(nsIInputStream** aResult,
+                                           uint64_t* aContentLength,
+                                           nsACString& aContentTypeWithCharset,
+                                           nsACString& aCharset) const
+{
+  return mBody->GetSendInfo(aResult, aContentLength, aContentTypeWithCharset,
+                            aCharset);
+}
+
+} // dom namespace
+} // mozilla namespace
diff --git a/dom/fetch/BodyExtractor.h b/dom/fetch/BodyExtractor.h
new file mode 100644
index 0000000000..7d4bfd068e
--- /dev/null
+++ b/dom/fetch/BodyExtractor.h
@@ -0,0 +1,46 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_BodyExtractor_h
+#define mozilla_dom_BodyExtractor_h
+
+#include "nsString.h"
+
+class nsIInputStream;
+
+namespace mozilla {
+namespace dom {
+
+class BodyExtractorBase
+{
+public:
+  virtual nsresult GetAsStream(nsIInputStream** aResult,
+                               uint64_t* aContentLength,
+                               nsACString& aContentTypeWithCharset,
+                               nsACString& aCharset) const = 0;
+};
+
+// The implementation versions of this template are:
+// ArrayBuffer, ArrayBufferView, Blob, FormData, nsAString, nsIDocument,
+// nsIInputStream, nsIXHRSendable, URLSearchParams
+template<typename Type>
+class BodyExtractor final : public BodyExtractorBase
+{
+  Type* mBody;
+public:
+  explicit BodyExtractor(Type* aBody) : mBody(aBody)
+  {}
+
+  nsresult GetAsStream(nsIInputStream** aResult,
+                       uint64_t* aContentLength,
+                       nsACString& aContentTypeWithCharset,
+                       nsACString& aCharset) const override;
+};
+
+} // dom namespace
+} // mozilla namespace
+
+#endif // mozilla_dom_BodyExtractor_h
diff --git a/dom/fetch/Fetch.cpp b/dom/fetch/Fetch.cpp
index 63e2c83eaa..de54b6094e 100644
--- a/dom/fetch/Fetch.cpp
+++ b/dom/fetch/Fetch.cpp
@@ -12,7 +12,6 @@
 #include "nsIStreamLoader.h"
 #include "nsIThreadRetargetableRequest.h"
 #include "nsIUnicodeDecoder.h"
-#include "nsIUnicodeEncoder.h"
 
 #include "nsDOMString.h"
 #include "nsNetUtil.h"
@@ -24,7 +23,6 @@
 #include "mozilla/ErrorResult.h"
 #include "mozilla/dom/BodyUtil.h"
 #include "mozilla/dom/DOMError.h"
-#include "mozilla/dom/EncodingUtils.h"
 #include "mozilla/dom/Exceptions.h"
 #include "mozilla/dom/FetchDriver.h"
 #include "mozilla/dom/File.h"
@@ -40,6 +38,7 @@
 #include "mozilla/dom/WorkerPrivate.h"
 #include "mozilla/dom/workers/ServiceWorkerManager.h"
 
+#include "BodyExtractor.h"
 #include "FetchObserver.h"
 #include "InternalRequest.h"
 #include "InternalResponse.h"
@@ -725,154 +724,48 @@ WorkerFetchResolver::FlushConsoleReport()
   mReporter->FlushConsoleReports(worker->GetDocument());
 }
 
-namespace {
-
-nsresult
-ExtractFromArrayBuffer(const ArrayBuffer& aBuffer,
-                       nsIInputStream** aStream,
-                       uint64_t& aContentLength)
-{
-  aBuffer.ComputeLengthAndData();
-  aContentLength = aBuffer.Length();
-  //XXXnsm reinterpret_cast<> is used in DOMParser, should be ok.
-  return NS_NewByteInputStream(aStream,
-                               reinterpret_cast<char*>(aBuffer.Data()),
-                               aBuffer.Length(), NS_ASSIGNMENT_COPY);
-}
-
-nsresult
-ExtractFromArrayBufferView(const ArrayBufferView& aBuffer,
-                           nsIInputStream** aStream,
-                           uint64_t& aContentLength)
-{
-  aBuffer.ComputeLengthAndData();
-  aContentLength = aBuffer.Length();
-  //XXXnsm reinterpret_cast<> is used in DOMParser, should be ok.
-  return NS_NewByteInputStream(aStream,
-                               reinterpret_cast<char*>(aBuffer.Data()),
-                               aBuffer.Length(), NS_ASSIGNMENT_COPY);
-}
-
-nsresult
-ExtractFromBlob(const Blob& aBlob,
-                nsIInputStream** aStream,
-                nsCString& aContentType,
-                uint64_t& aContentLength)
-{
-  RefPtr<BlobImpl> impl = aBlob.Impl();
-  ErrorResult rv;
-  aContentLength = impl->GetSize(rv);
-  if (NS_WARN_IF(rv.Failed())) {
-    return rv.StealNSResult();
-  }
-
-  impl->GetInternalStream(aStream, rv);
-  if (NS_WARN_IF(rv.Failed())) {
-    return rv.StealNSResult();
-  }
-
-  nsAutoString type;
-  impl->GetType(type);
-  aContentType = NS_ConvertUTF16toUTF8(type);
-  return NS_OK;
-}
-
-nsresult
-ExtractFromFormData(FormData& aFormData,
-                    nsIInputStream** aStream,
-                    nsCString& aContentType,
-                    uint64_t& aContentLength)
-{
-  nsAutoCString unusedCharset;
-  return aFormData.GetSendInfo(aStream, &aContentLength,
-                               aContentType, unusedCharset);
-}
-
-nsresult
-ExtractFromUSVString(const nsString& aStr,
-                     nsIInputStream** aStream,
-                     nsCString& aContentType,
-                     uint64_t& aContentLength)
-{
-  nsCOMPtr<nsIUnicodeEncoder> encoder = EncodingUtils::EncoderForEncoding("UTF-8");
-  if (!encoder) {
-    return NS_ERROR_OUT_OF_MEMORY;
-  }
-
-  int32_t destBufferLen;
-  nsresult rv = encoder->GetMaxLength(aStr.get(), aStr.Length(), &destBufferLen);
-  if (NS_WARN_IF(NS_FAILED(rv))) {
-    return rv;
-  }
-
-  nsCString encoded;
-  if (!encoded.SetCapacity(destBufferLen, fallible)) {
-    return NS_ERROR_OUT_OF_MEMORY;
-  }
-
-  char* destBuffer = encoded.BeginWriting();
-  int32_t srcLen = (int32_t) aStr.Length();
-  int32_t outLen = destBufferLen;
-  rv = encoder->Convert(aStr.get(), &srcLen, destBuffer, &outLen);
-  if (NS_WARN_IF(NS_FAILED(rv))) {
-    return rv;
-  }
-
-  MOZ_ASSERT(outLen <= destBufferLen);
-  encoded.SetLength(outLen);
-
-  aContentType = NS_LITERAL_CSTRING("text/plain;charset=UTF-8");
-  aContentLength = outLen;
-
-  return NS_NewCStringInputStream(aStream, encoded);
-}
-
-nsresult
-ExtractFromURLSearchParams(const URLSearchParams& aParams,
-                           nsIInputStream** aStream,
-                           nsCString& aContentType,
-                           uint64_t& aContentLength)
-{
-  nsAutoString serialized;
-  aParams.Stringify(serialized);
-  aContentType = NS_LITERAL_CSTRING("application/x-www-form-urlencoded;charset=UTF-8");
-  aContentLength = serialized.Length();
-  return NS_NewCStringInputStream(aStream, NS_ConvertUTF16toUTF8(serialized));
-}
-} // namespace
-
 nsresult
 ExtractByteStreamFromBody(const OwningArrayBufferOrArrayBufferViewOrBlobOrFormDataOrUSVStringOrURLSearchParams& aBodyInit,
                           nsIInputStream** aStream,
-                          nsCString& aContentType,
+                          nsCString& aContentTypeWithCharset,
                           uint64_t& aContentLength)
 {
   MOZ_ASSERT(aStream);
+  nsAutoCString charset;
+  aContentTypeWithCharset.SetIsVoid(true);
 
   if (aBodyInit.IsArrayBuffer()) {
-    const ArrayBuffer& buf = aBodyInit.GetAsArrayBuffer();
-    return ExtractFromArrayBuffer(buf, aStream, aContentLength);
+    BodyExtractor<const ArrayBuffer> body(&aBodyInit.GetAsArrayBuffer());
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsArrayBufferView()) {
-    const ArrayBufferView& buf = aBodyInit.GetAsArrayBufferView();
-    return ExtractFromArrayBufferView(buf, aStream, aContentLength);
+    BodyExtractor<const ArrayBufferView> body(&aBodyInit.GetAsArrayBufferView());
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsBlob()) {
-    const Blob& blob = aBodyInit.GetAsBlob();
-    return ExtractFromBlob(blob, aStream, aContentType, aContentLength);
+    Blob& blob = aBodyInit.GetAsBlob();
+    BodyExtractor<Blob> body(&blob);
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsFormData()) {
-    FormData& form = aBodyInit.GetAsFormData();
-    return ExtractFromFormData(form, aStream, aContentType, aContentLength);
+    FormData& formData = aBodyInit.GetAsFormData();
+    BodyExtractor<FormData> body(&formData);
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsUSVString()) {
-    nsAutoString str;
-    str.Assign(aBodyInit.GetAsUSVString());
-    return ExtractFromUSVString(str, aStream, aContentType, aContentLength);
+    BodyExtractor<const nsAString> body(&aBodyInit.GetAsUSVString());
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsURLSearchParams()) {
-    URLSearchParams& params = aBodyInit.GetAsURLSearchParams();
-    return ExtractFromURLSearchParams(params, aStream, aContentType, aContentLength);
+    URLSearchParams& usp = aBodyInit.GetAsURLSearchParams();
+    BodyExtractor<URLSearchParams> body(&usp);
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
 
   NS_NOTREACHED("Should never reach here");
@@ -882,36 +775,47 @@ ExtractByteStreamFromBody(const OwningArrayBufferOrArrayBufferViewOrBlobOrFormDa
 nsresult
 ExtractByteStreamFromBody(const ArrayBufferOrArrayBufferViewOrBlobOrFormDataOrUSVStringOrURLSearchParams& aBodyInit,
                           nsIInputStream** aStream,
-                          nsCString& aContentType,
+                          nsCString& aContentTypeWithCharset,
                           uint64_t& aContentLength)
 {
   MOZ_ASSERT(aStream);
   MOZ_ASSERT(!*aStream);
 
+  nsAutoCString charset;
+  aContentTypeWithCharset.SetIsVoid(true);
+
   if (aBodyInit.IsArrayBuffer()) {
-    const ArrayBuffer& buf = aBodyInit.GetAsArrayBuffer();
-    return ExtractFromArrayBuffer(buf, aStream, aContentLength);
+    BodyExtractor<const ArrayBuffer> body(&aBodyInit.GetAsArrayBuffer());
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsArrayBufferView()) {
-    const ArrayBufferView& buf = aBodyInit.GetAsArrayBufferView();
-    return ExtractFromArrayBufferView(buf, aStream, aContentLength);
+    BodyExtractor<const ArrayBufferView> body(&aBodyInit.GetAsArrayBufferView());
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsBlob()) {
-    const Blob& blob = aBodyInit.GetAsBlob();
-    return ExtractFromBlob(blob, aStream, aContentType, aContentLength);
+    Blob& blob = aBodyInit.GetAsBlob();
+    BodyExtractor<Blob> body(&blob);
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsFormData()) {
-    FormData& form = aBodyInit.GetAsFormData();
-    return ExtractFromFormData(form, aStream, aContentType, aContentLength);
+    FormData& formData = aBodyInit.GetAsFormData();
+    BodyExtractor<FormData> body(&formData);
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsUSVString()) {
-    nsAutoString str;
-    str.Assign(aBodyInit.GetAsUSVString());
-    return ExtractFromUSVString(str, aStream, aContentType, aContentLength);
+    BodyExtractor<const nsAString> body(&aBodyInit.GetAsUSVString());
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
   if (aBodyInit.IsURLSearchParams()) {
-    URLSearchParams& params = aBodyInit.GetAsURLSearchParams();
-    return ExtractFromURLSearchParams(params, aStream, aContentType, aContentLength);
+    URLSearchParams& usp = aBodyInit.GetAsURLSearchParams();
+    BodyExtractor<URLSearchParams> body(&usp);
+    return body.GetAsStream(aStream, &aContentLength, aContentTypeWithCharset,
+                            charset);
   }
 
   NS_NOTREACHED("Should never reach here");
diff --git a/dom/fetch/Request.cpp b/dom/fetch/Request.cpp
index 47fb62e885..4d9f6fc342 100644
--- a/dom/fetch/Request.cpp
+++ b/dom/fetch/Request.cpp
@@ -580,11 +580,11 @@ Request::Constructor(const GlobalObject& aGlobal,
       const OwningArrayBufferOrArrayBufferViewOrBlobOrFormDataOrUSVStringOrURLSearchParams& bodyInit =
         bodyInitNullable.Value();
       nsCOMPtr<nsIInputStream> stream;
-      nsAutoCString contentType;
+      nsAutoCString contentTypeWithCharset;
       uint64_t contentLengthUnused;
       aRv = ExtractByteStreamFromBody(bodyInit,
                                       getter_AddRefs(stream),
-                                      contentType,
+                                      contentTypeWithCharset,
                                       contentLengthUnused);
       if (NS_WARN_IF(aRv.Failed())) {
         return nullptr;
@@ -592,10 +592,10 @@ Request::Constructor(const GlobalObject& aGlobal,
 
       temporaryBody = stream;
 
-      if (!contentType.IsVoid() &&
+      if (!contentTypeWithCharset.IsVoid() &&
           !requestHeaders->Has(NS_LITERAL_CSTRING("Content-Type"), aRv)) {
         requestHeaders->Append(NS_LITERAL_CSTRING("Content-Type"),
-                               contentType, aRv);
+                               contentTypeWithCharset, aRv);
       }
 
       if (NS_WARN_IF(aRv.Failed())) {
diff --git a/dom/fetch/Response.cpp b/dom/fetch/Response.cpp
index 20487906ba..337139e3d2 100644
--- a/dom/fetch/Response.cpp
+++ b/dom/fetch/Response.cpp
@@ -224,22 +224,23 @@ Response::Constructor(const GlobalObject& aGlobal,
     }
 
     nsCOMPtr<nsIInputStream> bodyStream;
-    nsCString contentType;
+    nsCString contentTypeWithCharset;
     uint64_t bodySize = 0;
     aRv = ExtractByteStreamFromBody(aBody.Value().Value(),
                                     getter_AddRefs(bodyStream),
-                                    contentType,
+                                    contentTypeWithCharset,
                                     bodySize);
     if (NS_WARN_IF(aRv.Failed())) {
       return nullptr;
     }
     internalResponse->SetBody(bodyStream, bodySize);
 
-    if (!contentType.IsVoid() &&
+    if (!contentTypeWithCharset.IsVoid() &&
         !internalResponse->Headers()->Has(NS_LITERAL_CSTRING("Content-Type"), aRv)) {
       // Ignore Append() failing here.
       ErrorResult error;
-      internalResponse->Headers()->Append(NS_LITERAL_CSTRING("Content-Type"), contentType, error);
+      internalResponse->Headers()->Append(NS_LITERAL_CSTRING("Content-Type"),
+                                          contentTypeWithCharset, error);
       error.SuppressException();
     }
 
diff --git a/dom/fetch/moz.build b/dom/fetch/moz.build
index cd41620f93..d08bdf0cf2 100644
--- a/dom/fetch/moz.build
+++ b/dom/fetch/moz.build
@@ -4,6 +4,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 EXPORTS.mozilla.dom += [
+    'BodyExtractor.h',
     'ChannelInfo.h',
     'Fetch.h',
     'FetchDriver.h',
@@ -19,6 +20,7 @@ EXPORTS.mozilla.dom += [
 ]
 
 UNIFIED_SOURCES += [
+    'BodyExtractor.cpp',
     'ChannelInfo.cpp',
     'Fetch.cpp',
     'FetchConsumer.cpp',
diff --git a/dom/tests/mochitest/beacon/beacon-handler.sjs b/dom/tests/mochitest/beacon/beacon-handler.sjs
index 4c51d371d8..e4a98f7202 100644
--- a/dom/tests/mochitest/beacon/beacon-handler.sjs
+++ b/dom/tests/mochitest/beacon/beacon-handler.sjs
@@ -88,7 +88,8 @@ function handleRequest(request, response) {
       data += charcode;
     }
 
-    var mimetype = request.getHeader("Content-Type");
+    var mimetype = request.hasHeader("Content-Type")
+      ? request.getHeader("Content-Type") : "application/octet-stream";
 
     // check to see if this is form data.
     if (mimetype.indexOf("multipart/form-data") != -1) {
diff --git a/dom/url/URLSearchParams.cpp b/dom/url/URLSearchParams.cpp
index 3a0311aabd..19cc3cda6b 100644
--- a/dom/url/URLSearchParams.cpp
+++ b/dom/url/URLSearchParams.cpp
@@ -575,9 +575,10 @@ URLSearchParams::ReadStructuredClone(JSStructuredCloneReader* aReader)
 
 NS_IMETHODIMP
 URLSearchParams::GetSendInfo(nsIInputStream** aBody, uint64_t* aContentLength,
-                             nsACString& aContentType, nsACString& aCharset)
+                             nsACString& aContentTypeWithCharset,
+                             nsACString& aCharset)
 {
-  aContentType.AssignLiteral("application/x-www-form-urlencoded");
+  aContentTypeWithCharset.AssignLiteral("application/x-www-form-urlencoded;charset=UTF-8");
   aCharset.AssignLiteral("UTF-8");
 
   nsAutoString serialized;
diff --git a/dom/webidl/Navigator.webidl b/dom/webidl/Navigator.webidl
index 43d53cbac0..37c853da94 100644
--- a/dom/webidl/Navigator.webidl
+++ b/dom/webidl/Navigator.webidl
@@ -294,7 +294,7 @@ partial interface Navigator {
 partial interface Navigator {
   [Throws, Pref="beacon.enabled"]
   boolean sendBeacon(DOMString url,
-                     optional (ArrayBufferView or Blob or DOMString or FormData)? data = null);
+                     optional BodyInit? data = null);
 };
 
 partial interface Navigator {
diff --git a/dom/xhr/XMLHttpRequestMainThread.cpp b/dom/xhr/XMLHttpRequestMainThread.cpp
index c7a4f6e6aa..48eb494747 100644
--- a/dom/xhr/XMLHttpRequestMainThread.cpp
+++ b/dom/xhr/XMLHttpRequestMainThread.cpp
@@ -2298,175 +2298,6 @@ XMLHttpRequestMainThread::ChangeStateToDone()
   }
 }
 
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<nsIDocument>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  nsCOMPtr<nsIDOMDocument> domdoc(do_QueryInterface(mBody));
-  NS_ENSURE_STATE(domdoc);
-  aCharset.AssignLiteral("UTF-8");
-
-  nsresult rv;
-  nsCOMPtr<nsIStorageStream> storStream;
-  rv = NS_NewStorageStream(4096, UINT32_MAX, getter_AddRefs(storStream));
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  nsCOMPtr<nsIOutputStream> output;
-  rv = storStream->GetOutputStream(0, getter_AddRefs(output));
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  if (mBody->IsHTMLDocument()) {
-    aContentType.AssignLiteral("text/html");
-
-    nsString serialized;
-    if (!nsContentUtils::SerializeNodeToMarkup(mBody, true, serialized)) {
-      return NS_ERROR_OUT_OF_MEMORY;
-    }
-
-    nsAutoCString utf8Serialized;
-    if (!AppendUTF16toUTF8(serialized, utf8Serialized, fallible)) {
-      return NS_ERROR_OUT_OF_MEMORY;
-    }
-
-    uint32_t written;
-    rv = output->Write(utf8Serialized.get(), utf8Serialized.Length(), &written);
-    NS_ENSURE_SUCCESS(rv, rv);
-
-    MOZ_ASSERT(written == utf8Serialized.Length());
-  } else {
-    aContentType.AssignLiteral("application/xml");
-
-    nsCOMPtr<nsIDOMSerializer> serializer =
-      do_CreateInstance(NS_XMLSERIALIZER_CONTRACTID, &rv);
-    NS_ENSURE_SUCCESS(rv, rv);
-
-    // Make sure to use the encoding we'll send
-    rv = serializer->SerializeToStream(domdoc, output, aCharset);
-    NS_ENSURE_SUCCESS(rv, rv);
-  }
-
-  output->Close();
-
-  uint32_t length;
-  rv = storStream->GetLength(&length);
-  NS_ENSURE_SUCCESS(rv, rv);
-  *aContentLength = length;
-
-  rv = storStream->NewInputStream(0, aResult);
-  NS_ENSURE_SUCCESS(rv, rv);
-  return NS_OK;
-}
-
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<const nsAString>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  aContentType.AssignLiteral("text/plain");
-  aCharset.AssignLiteral("UTF-8");
-
-  nsAutoCString converted;
-  if (!AppendUTF16toUTF8(*mBody, converted, fallible)) {
-    return NS_ERROR_OUT_OF_MEMORY;
-  }
-
-  *aContentLength = converted.Length();
-  nsresult rv = NS_NewCStringInputStream(aResult, converted);
-  NS_ENSURE_SUCCESS(rv, rv);
-  return NS_OK;
-}
-
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<nsIInputStream>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  aContentType.AssignLiteral("text/plain");
-  aCharset.Truncate();
-
-  nsresult rv = mBody->Available(aContentLength);
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  nsCOMPtr<nsIInputStream> stream(mBody);
-  stream.forget(aResult);
-  return NS_OK;
-}
-
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<Blob>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  return mBody->GetSendInfo(aResult, aContentLength, aContentType, aCharset);
-}
-
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<FormData>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  return mBody->GetSendInfo(aResult, aContentLength, aContentType, aCharset);
-}
-
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<URLSearchParams>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  return mBody->GetSendInfo(aResult, aContentLength, aContentType, aCharset);
-}
-
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<nsIXHRSendable>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  return mBody->GetSendInfo(aResult, aContentLength, aContentType, aCharset);
-}
-
-static nsresult
-GetBufferDataAsStream(const uint8_t* aData, uint32_t aDataLength,
-                      nsIInputStream** aResult, uint64_t* aContentLength,
-                      nsACString& aContentType, nsACString& aCharset)
-{
-  aContentType.SetIsVoid(true);
-  aCharset.Truncate();
-
-  *aContentLength = aDataLength;
-  const char* data = reinterpret_cast<const char*>(aData);
-
-  nsCOMPtr<nsIInputStream> stream;
-  nsresult rv = NS_NewByteInputStream(getter_AddRefs(stream), data, aDataLength,
-                                      NS_ASSIGNMENT_COPY);
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  stream.forget(aResult);
-
-  return NS_OK;
-}
-
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<const ArrayBuffer>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  mBody->ComputeLengthAndData();
-  return GetBufferDataAsStream(mBody->Data(), mBody->Length(),
-                               aResult, aContentLength, aContentType, aCharset);
-}
-
-template<> nsresult
-XMLHttpRequestMainThread::RequestBody<const ArrayBufferView>::GetAsStream(
-   nsIInputStream** aResult, uint64_t* aContentLength,
-   nsACString& aContentType, nsACString& aCharset) const
-{
-  mBody->ComputeLengthAndData();
-  return GetBufferDataAsStream(mBody->Data(), mBody->Length(),
-                               aResult, aContentLength, aContentType, aCharset);
-}
-
-
 nsresult
 XMLHttpRequestMainThread::CreateChannel()
 {
@@ -2793,7 +2624,7 @@ XMLHttpRequestMainThread::Send(nsIVariant* aVariant)
     // document?
     nsCOMPtr<nsIDocument> doc = do_QueryInterface(supports);
     if (doc) {
-      RequestBody<nsIDocument> body(doc);
+      BodyExtractor<nsIDocument> body(doc);
       return SendInternal(&body);
     }
 
@@ -2802,21 +2633,21 @@ XMLHttpRequestMainThread::Send(nsIVariant* aVariant)
     if (wstr) {
       nsAutoString string;
       wstr->GetData(string);
-      RequestBody<const nsAString> body(&string);
+      BodyExtractor<const nsAString> body(&string);
       return SendInternal(&body);
     }
 
     // nsIInputStream?
     nsCOMPtr<nsIInputStream> stream = do_QueryInterface(supports);
     if (stream) {
-      RequestBody<nsIInputStream> body(stream);
+      BodyExtractor<nsIInputStream> body(stream);
       return SendInternal(&body);
     }
 
     // nsIXHRSendable?
     nsCOMPtr<nsIXHRSendable> sendable = do_QueryInterface(supports);
     if (sendable) {
-      RequestBody<nsIXHRSendable> body(sendable);
+      BodyExtractor<nsIXHRSendable> body(sendable);
       return SendInternal(&body);
     }
 
@@ -2829,7 +2660,7 @@ XMLHttpRequestMainThread::Send(nsIVariant* aVariant)
       JS::Rooted<JSObject*> obj(rootingCx, realVal.toObjectOrNull());
       RootedSpiderMonkeyInterface<ArrayBuffer> buf(rootingCx);
       if (buf.Init(obj)) {
-        RequestBody<const ArrayBuffer> body(&buf);
+        BodyExtractor<const ArrayBuffer> body(&buf);
         return SendInternal(&body);
       }
     }
@@ -2846,12 +2677,12 @@ XMLHttpRequestMainThread::Send(nsIVariant* aVariant)
   nsString string;
   string.Adopt(data, len);
 
-  RequestBody<const nsAString> body(&string);
+  BodyExtractor<const nsAString> body(&string);
   return SendInternal(&body);
 }
 
 nsresult
-XMLHttpRequestMainThread::SendInternal(const RequestBodyBase* aBody)
+XMLHttpRequestMainThread::SendInternal(const BodyExtractorBase* aBody)
 {
   NS_ENSURE_TRUE(mPrincipal, NS_ERROR_NOT_INITIALIZED);
 
@@ -2917,13 +2748,6 @@ XMLHttpRequestMainThread::SendInternal(const RequestBodyBase* aBody)
       mAuthorRequestHeaders.Get("content-type", uploadContentType);
       if (uploadContentType.IsVoid()) {
         uploadContentType = defaultContentType;
-
-        if (!charset.IsEmpty()) {
-          // If we are providing the default content type, then we also need to
-          // provide a charset declaration.
-          uploadContentType.Append(NS_LITERAL_CSTRING(";charset="));
-          uploadContentType.Append(charset);
-        }
       }
 
       // We don't want to set a charset for streams.
diff --git a/dom/xhr/XMLHttpRequestMainThread.h b/dom/xhr/XMLHttpRequestMainThread.h
index 4a6eeec2ef..ed02fd1da4 100644
--- a/dom/xhr/XMLHttpRequestMainThread.h
+++ b/dom/xhr/XMLHttpRequestMainThread.h
@@ -34,6 +34,7 @@
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/NotNull.h"
 #include "mozilla/dom/MutableBlobStorage.h"
+#include "mozilla/dom/BodyExtractor.h"
 #include "mozilla/dom/TypedArray.h"
 #include "mozilla/dom/XMLHttpRequest.h"
 #include "mozilla/dom/XMLHttpRequestBinding.h"
@@ -288,34 +289,7 @@ public:
 private:
   virtual ~XMLHttpRequestMainThread();
 
-  class RequestBodyBase
-  {
-  public:
-    virtual nsresult GetAsStream(nsIInputStream** aResult,
-                                 uint64_t* aContentLength,
-                                 nsACString& aContentType,
-                                 nsACString& aCharset) const
-    {
-      NS_ASSERTION(false, "RequestBodyBase should not be used directly.");
-      return NS_ERROR_FAILURE;
-    }
-  };
-
-  template<typename Type>
-  class RequestBody final : public RequestBodyBase
-  {
-    Type* mBody;
-  public:
-    explicit RequestBody(Type* aBody) : mBody(aBody)
-    {
-    }
-    nsresult GetAsStream(nsIInputStream** aResult,
-                         uint64_t* aContentLength,
-                         nsACString& aContentType,
-                         nsACString& aCharset) const override;
-  };
-
-  nsresult SendInternal(const RequestBodyBase* aBody);
+  nsresult SendInternal(const BodyExtractorBase* aBody);
 
   bool IsCrossSiteCORSRequest() const;
   bool IsDeniedCrossSiteCORSRequest();
@@ -336,7 +310,7 @@ public:
   Send(JSContext* /*aCx*/, const ArrayBuffer& aArrayBuffer,
        ErrorResult& aRv) override
   {
-    RequestBody<const ArrayBuffer> body(&aArrayBuffer);
+    BodyExtractor<const ArrayBuffer> body(&aArrayBuffer);
     aRv = SendInternal(&body);
   }
 
@@ -344,28 +318,28 @@ public:
   Send(JSContext* /*aCx*/, const ArrayBufferView& aArrayBufferView,
        ErrorResult& aRv) override
   {
-    RequestBody<const ArrayBufferView> body(&aArrayBufferView);
+    BodyExtractor<const ArrayBufferView> body(&aArrayBufferView);
     aRv = SendInternal(&body);
   }
 
   virtual void
   Send(JSContext* /*aCx*/, Blob& aBlob, ErrorResult& aRv) override
   {
-    RequestBody<Blob> body(&aBlob);
+    BodyExtractor<Blob> body(&aBlob);
     aRv = SendInternal(&body);
   }
 
   virtual void Send(JSContext* /*aCx*/, URLSearchParams& aURLSearchParams,
                     ErrorResult& aRv) override
   {
-    RequestBody<URLSearchParams> body(&aURLSearchParams);
+    BodyExtractor<URLSearchParams> body(&aURLSearchParams);
     aRv = SendInternal(&body);
   }
 
   virtual void
   Send(JSContext* /*aCx*/, nsIDocument& aDoc, ErrorResult& aRv) override
   {
-    RequestBody<nsIDocument> body(&aDoc);
+    BodyExtractor<nsIDocument> body(&aDoc);
     aRv = SendInternal(&body);
   }
 
@@ -375,7 +349,7 @@ public:
     if (DOMStringIsNull(aString)) {
       Send(aCx, aRv);
     } else {
-      RequestBody<const nsAString> body(&aString);
+      BodyExtractor<const nsAString> body(&aString);
       aRv = SendInternal(&body);
     }
   }
@@ -383,7 +357,7 @@ public:
   virtual void
   Send(JSContext* /*aCx*/, FormData& aFormData, ErrorResult& aRv) override
   {
-    RequestBody<FormData> body(&aFormData);
+    BodyExtractor<FormData> body(&aFormData);
     aRv = SendInternal(&body);
   }
 
@@ -391,7 +365,7 @@ public:
   Send(JSContext* aCx, nsIInputStream* aStream, ErrorResult& aRv) override
   {
     NS_ASSERTION(aStream, "Null should go to string version");
-    RequestBody<nsIInputStream> body(aStream);
+    BodyExtractor<nsIInputStream> body(aStream);
     aRv = SendInternal(&body);
   }
 
diff --git a/dom/xhr/nsIXMLHttpRequest.idl b/dom/xhr/nsIXMLHttpRequest.idl
index 53e80bab70..5505bd47ee 100644
--- a/dom/xhr/nsIXMLHttpRequest.idl
+++ b/dom/xhr/nsIXMLHttpRequest.idl
@@ -326,9 +326,12 @@ interface nsIXMLHttpRequest : nsISupports
 
 [uuid(840d0d00-e83e-4a29-b3c7-67e96e90a499)]
 interface nsIXHRSendable : nsISupports {
+  // contentTypeWithCharset can be set to the contentType or
+  // contentType+charset based on what the spec says.
+  // See: https://fetch.spec.whatwg.org/#concept-bodyinit-extract
   void getSendInfo(out nsIInputStream body,
                    out uint64_t contentLength,
-                   out ACString contentType,
+                   out ACString contentTypeWithCharset,
                    out ACString charset);
 };