--- a/toolkit/components/mediasniffer/nsMediaSniffer.cpp
+++ b/toolkit/components/mediasniffer/nsMediaSniffer.cpp
@@ -39,16 +39,17 @@ nsMediaSnifferEntry nsMediaSniffer::sSni
 // For a complete list of file types, see http://www.ftyps.com/index.html
 nsMediaSnifferEntry sFtypEntries[] = {
   PATTERN_ENTRY("\xFF\xFF\xFF", "mp4", VIDEO_MP4), // Could be mp41 or mp42.
   PATTERN_ENTRY("\xFF\xFF\xFF", "avc", VIDEO_MP4), // Could be avc1, avc2, ...
   PATTERN_ENTRY("\xFF\xFF\xFF", "3gp", VIDEO_3GPP), // Could be 3gp4, 3gp5, ...
   PATTERN_ENTRY("\xFF\xFF\xFF\xFF", "M4A ", AUDIO_MP4),
   PATTERN_ENTRY("\xFF\xFF\xFF\xFF", "M4P ", AUDIO_MP4),
   PATTERN_ENTRY("\xFF\xFF\xFF\xFF", "qt  ", VIDEO_QUICKTIME),
+  PATTERN_ENTRY("\xFF\xFF\xFF", "iso", VIDEO_MP4), // Could be isom or iso2.
 };
 
 static bool MatchesBrands(const uint8_t aData[4], nsACString& aSniffedType)
 {
   for (size_t i = 0; i < mozilla::ArrayLength(sFtypEntries); ++i) {
     const auto& currentEntry = sFtypEntries[i];
     bool matched = true;
     MOZ_ASSERT(currentEntry.mLength <= 4, "Pattern is too large to match brand strings.");