--- a/content/html/content/src/nsHTMLMediaElement.cpp
+++ b/content/html/content/src/nsHTMLMediaElement.cpp
@@ -1067,17 +1067,18 @@ nsresult nsHTMLMediaElement::LoadResourc
     channelPolicy->SetLoadType(nsIContentPolicy::TYPE_MEDIA);
   }
   nsCOMPtr<nsIChannel> channel;
   rv = NS_NewChannel(getter_AddRefs(channel),
                      mLoadingSrc,
                      nullptr,
                      loadGroup,
                      nullptr,
-                     nsICachingChannel::LOAD_BYPASS_LOCAL_CACHE_IF_BUSY,
+                     nsICachingChannel::LOAD_BYPASS_LOCAL_CACHE_IF_BUSY |
+                     nsIChannel::LOAD_TREAT_APPLICATION_OCTET_STREAM_AS_UNKNOWN,
                      channelPolicy);
   NS_ENSURE_SUCCESS(rv,rv);
 
   // The listener holds a strong reference to us.  This creates a
   // reference cycle, once we've set mChannel, which is manually broken
   // in the listener's OnStartRequest method after it is finished with
   // the element. The cycle will also be broken if we get a shutdown
   // notification before OnStartRequest fires.  Necko guarantees that

--- a/content/media/test/Makefile.in
+++ b/content/media/test/Makefile.in
@@ -122,16 +122,18 @@ MOCHITEST_FILES = \
 		test_autoplay_contentEditable.html \
 		test_buffered.html \
 		test_bug448534.html \
 		test_bug463162.xhtml \
 		test_decoder_disable.html \
 		test_media_selection.html \
 		test_playback.html \
 		test_seekLies.html \
+		test_media_sniffer.html \
+		contentType.sjs \
 		$(NULL)
 
 $(warning test_error_in_video_document.html is disabled for intermittent failures. Bug 608634)
 
 # Disabled on Windows for frequent intermittent failures
 ifneq ($(OS_ARCH), WINNT)
 MOCHITEST_FILES += \
 		test_streams_element_capture.html \

--- a/netwerk/mime/nsMimeTypes.h
+++ b/netwerk/mime/nsMimeTypes.h
@@ -70,16 +70,17 @@
 #define APPLICATION_XSLT_XML                "application/xslt+xml"
 #define APPLICATION_MATHML_XML              "application/mathml+xml"
 #define APPLICATION_RDF_XML                 "application/rdf+xml"
 
 #define AUDIO_BASIC                         "audio/basic"
 #define AUDIO_OGG                           "audio/ogg"
 #define AUDIO_WAV                           "audio/x-wav"
 #define AUDIO_WEBM                          "audio/webm"
+#define AUDIO_MP3                           "audio/mpeg"
 
 #define BINARY_OCTET_STREAM                 "binary/octet-stream"
 
 #define IMAGE_GIF                           "image/gif"
 #define IMAGE_JPG                           "image/jpeg"
 #define IMAGE_PJPG                          "image/pjpeg"
 #define IMAGE_PNG                           "image/png"
 #define IMAGE_PPM                           "image/x-portable-pixmap"

--- a/toolkit/components/Makefile.in
+++ b/toolkit/components/Makefile.in
@@ -23,16 +23,17 @@ PARALLEL_DIRS += \
   console \
   contentprefs \
   cookie \
   downloads \
   exthelper \
   filepicker \
   find \
   intl \
+  mediasniffer \
   microformats \
   osfile \
   parentalcontrols \
   passwordmgr \
   perf \
   places \
   prompts \
   reflect \

new file mode 100644
--- /dev/null
+++ b/toolkit/components/mediasniffer/Makefile.in
@@ -0,0 +1,30 @@
+# 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/.
+
+DEPTH = ../../..
+topsrcdir = @top_srcdir@
+srcdir = @srcdir@
+VPATH = @srcdir@
+
+include $(DEPTH)/config/autoconf.mk
+
+MODULE = mediasniffer
+LIBRARY_NAME = mediasniffer
+LIBXUL_LIBRARY  = 1
+EXPORT_LIBRARY = 1
+MODULE_NAME = nsMediaSnifferModule
+IS_COMPONENT = 1
+
+
+CPPSRCS = \
+    nsMediaSniffer.cpp \
+    nsMediaSnifferModule.cpp \
+    $(NULL)
+
+EXPORTS = \
+    nsMediaSniffer.h \
+    $(NULL)
+
+include $(topsrcdir)/config/config.mk
+include $(topsrcdir)/config/rules.mk

new file mode 100644
--- /dev/null
+++ b/toolkit/components/mediasniffer/nsMediaSniffer.cpp
@@ -0,0 +1,107 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 tw=80 et cindent: */
+/* 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 "nsMediaSniffer.h"
+#include "nsMemory.h"
+#include "nsIHttpChannel.h"
+#include "nsAString.h"
+#include "nsMimeTypes.h"
+#include "mozilla/ModuleUtils.h"
+
+#include "nsIClassInfoImpl.h"
+
+// The minimum number of bytes that are needed to attempt to sniff an mp4 file.
+static const unsigned MP4_MIN_BYTES_COUNT = 12;
+// The maximum number of bytes to consider when attempting to sniff a file.
+static const PRUint32 MAX_BYTES_SNIFFED = 512;
+
+NS_IMPL_CLASSINFO(nsMediaSniffer, NULL, 0, NS_MEDIA_SNIFFER_CID)
+NS_IMPL_ISUPPORTS1(nsMediaSniffer, nsIContentSniffer)
+
+nsMediaSniffer::nsMediaSnifferEntry nsMediaSniffer::sSnifferEntries[] = {
+  // The string OggS, followed by the null byte.
+  PATTERN_ENTRY("\xFF\xFF\xFF\xFF\xFF", "OggS", APPLICATION_OGG),
+  // The string RIFF, followed by four bytes, followed by the string WAVE
+  PATTERN_ENTRY("\xFF\xFF\xFF\xFF\x00\x00\x00\x00\xFF\xFF\xFF\xFF", "RIFF\x00\x00\x00\x00WAVE", AUDIO_WAV),
+  // WebM
+  PATTERN_ENTRY("\xFF\xFF\xFF\xFF", "\x1A\x45\xDF\xA3", VIDEO_WEBM),
+  // mp3 without ID3 tags.
+  PATTERN_ENTRY("\xFF\xFB", "\xFF\xFA", AUDIO_MP3),
+  // mp3 with ID3 tags, the string "ID3".
+  PATTERN_ENTRY("\xFF\xFF\xFF", "ID3", AUDIO_MP3)
+};
+
+// This function implements mp4 sniffing algorithm, described at
+// http://mimesniff.spec.whatwg.org/#signature-for-mp4
+static bool MatchesMP4(const PRUint8* aData, const PRUint32 aLength)
+{
+  if (aLength <= MP4_MIN_BYTES_COUNT) {
+    return false;
+  }
+  // Conversion from big endian to host byte order.
+  PRUint32 boxSize = (PRUint32)(aData[3] | aData[2] << 8 | aData[1] << 16 | aData[0] << 24);
+
+  // Boxsize should be evenly divisible by 4.
+  if (boxSize % 4 || aLength < boxSize) {
+    return false;
+  }
+  // The string "ftyp".
+  if (aData[4] != 0x66 ||
+      aData[5] != 0x74 ||
+      aData[6] != 0x79 ||
+      aData[7] != 0x70) {
+    return false;
+  }
+  for (PRUint32 i = 2; i <= boxSize / 4 - 1 ; i++) {
+    if (i == 3) {
+      continue;
+    }
+    // The string "mp4".
+    if (aData[4*i]   == 0x6D &&
+        aData[4*i+1] == 0x70 &&
+        aData[4*i+2] == 0x34) {
+      return true;
+    }
+  }
+  return false;
+}
+
+NS_IMETHODIMP
+nsMediaSniffer::GetMIMETypeFromContent(nsIRequest* aRequest,
+                                       const PRUint8* aData,
+                                       const PRUint32 aLength,
+                                       nsACString& aSniffedType)
+{
+  const PRUint32 clampedLength = NS_MIN(aLength, MAX_BYTES_SNIFFED);
+
+  for (PRUint32 i = 0; i < NS_ARRAY_LENGTH(sSnifferEntries); ++i) {
+    const nsMediaSnifferEntry& currentEntry = sSnifferEntries[i];
+    if (clampedLength < currentEntry.mLength || currentEntry.mLength == 0) {
+      continue;
+    }
+    bool matched = true;
+    for (PRUint32 j = 0; j < currentEntry.mLength; ++j) {
+      if ((currentEntry.mMask[j] & aData[j]) != currentEntry.mPattern[j]) {
+        matched = false;
+        break;
+      }
+    }
+    if (matched) {
+      aSniffedType.AssignASCII(currentEntry.mContentType);
+      return NS_OK;
+    }
+  }
+
+  if (MatchesMP4(aData, clampedLength)) {
+    aSniffedType.AssignLiteral(VIDEO_MP4);
+    return NS_OK;
+  }
+
+  // Could not sniff the media type, we are required to set it to
+  // application/octet-stream.
+  aSniffedType.AssignLiteral(APPLICATION_OCTET_STREAM);
+  return NS_ERROR_NOT_AVAILABLE;
+}

new file mode 100644
--- /dev/null
+++ b/toolkit/components/mediasniffer/nsMediaSniffer.h
@@ -0,0 +1,41 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 tw=80 et cindent: */
+/* 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 "nsIModule.h"
+#include "nsIFactory.h"
+
+#include "nsIComponentManager.h"
+#include "nsIComponentRegistrar.h"
+#include "nsIContentSniffer.h"
+
+// ed905ba3-c656-480e-934e-6bc35bd36aff
+#define NS_MEDIA_SNIFFER_CID \
+{0x3fdd6c28, 0x5b87, 0x4e3e, \
+{0x8b, 0x57, 0x8e, 0x83, 0xc2, 0x3c, 0x1a, 0x6d}}
+
+#define NS_MEDIA_SNIFFER_CONTRACTID "@mozilla.org/media/sniffer;1"
+
+class nsMediaSniffer : public nsIContentSniffer
+{
+  public:
+    NS_DECL_ISUPPORTS
+    NS_DECL_NSICONTENTSNIFFER
+  protected:
+    ~nsMediaSniffer() {};
+
+#define PATTERN_ENTRY(mask, pattern, contentType) \
+    {(const PRUint8*)mask, (const PRUint8*)pattern, sizeof(mask) - 1, contentType}
+
+  struct nsMediaSnifferEntry {
+    const PRUint8* mMask;
+    const PRUint8* mPattern;
+    const PRUint32 mLength;
+    const char* mContentType;
+  };
+
+  static nsMediaSnifferEntry sSnifferEntries[];
+};

new file mode 100644
--- /dev/null
+++ b/toolkit/components/mediasniffer/nsMediaSnifferModule.cpp
@@ -0,0 +1,37 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 "mozilla/ModuleUtils.h"
+
+#include "nsMediaSniffer.h"
+
+NS_GENERIC_FACTORY_CONSTRUCTOR(nsMediaSniffer)
+
+NS_DEFINE_NAMED_CID(NS_MEDIA_SNIFFER_CID);
+
+static const mozilla::Module::CIDEntry kMediaSnifferCIDs[] = {
+    { &kNS_MEDIA_SNIFFER_CID, false, NULL, nsMediaSnifferConstructor },
+    { NULL }
+};
+
+static const mozilla::Module::ContractIDEntry kMediaSnifferContracts[] = {
+    { NS_MEDIA_SNIFFER_CONTRACTID, &kNS_MEDIA_SNIFFER_CID },
+    { NULL }
+};
+
+static const mozilla::Module::CategoryEntry kMediaSnifferCategories[] = {
+    { "content-sniffing-services", NS_MEDIA_SNIFFER_CONTRACTID, NS_MEDIA_SNIFFER_CONTRACTID},
+    { "net-content-sniffers", NS_MEDIA_SNIFFER_CONTRACTID, NS_MEDIA_SNIFFER_CONTRACTID},
+    { NULL }
+};
+
+static const mozilla::Module kMediaSnifferModule = {
+    mozilla::Module::kVersion,
+    kMediaSnifferCIDs,
+    kMediaSnifferContracts,
+    kMediaSnifferCategories
+};
+
+NSMODULE_DEFN(nsMediaSnifferModule) = &kMediaSnifferModule;

--- a/toolkit/library/Makefile.in
+++ b/toolkit/library/Makefile.in
@@ -142,16 +142,17 @@ COMPONENT_LIBS += \
   i18n \
   chardet \
   jar$(VERSION_NUMBER) \
   startupcache \
   pref \
   htmlpars \
   identity \
   imglib2 \
+  mediasniffer \
   gkgfx \
   gklayout \
   docshell \
   embedcomponents \
   webbrwsr \
   nsappshell \
   txmgr \
   commandlines \

--- a/toolkit/library/nsStaticXULComponents.cpp
+++ b/toolkit/library/nsStaticXULComponents.cpp
@@ -175,16 +175,17 @@
     MODULE(nsJarModule)                      \
     ZIPWRITER_MODULE                         \
     MODULE(StartupCacheModule)               \
     MODULE(nsPrefModule)                     \
     MODULE(nsRDFModule)                      \
     MODULE(nsWindowDataSourceModule)         \
     MODULE(nsParserModule)                   \
     MODULE(nsImageLib2Module)                \
+    MODULE(nsMediaSnifferModule)             \
     MODULE(nsGfxModule)                      \
     PROFILER_MODULE                          \
     WIDGET_MODULES                           \
     ICON_MODULE                              \
     MODULE(nsPluginModule)                   \
     MODULE(nsLayoutModule)                   \
     MODULE(docshell_provider)                \
     MODULE(embedcomponents)                  \