Currently, we only check the mime type when determining whether to play back a downloaded video in Chrome. For certain unsupported codecs, such as mpeg4, this means that the video playback will fail. We need to check the codec prior to, or at the time of, playback and if Chrome doesn't support the codec, then we should intent out.

See https://bugs.chromium.org/p/chromium/issues/detail?id=657594 for more context.