If file extensions are used to determine the media type, I think the mapping of file extensions to particular media types should be strongly specified.
Consider the following Markdown:
Generally, this would refer to the music video of “everyhome” because of the file extension .mp4. However, the .mp4 extension can also be used for audio - the file extension can be used for AAC encoded music in iTunes, for example (even though .m4a is the norm for audio). So the file could just contain the audio of “everyhome” or both audio and video. If the implementation developer chooses the render it using the HTML
<audio> tag (and the file is indeed video), the video will fail to display. This is a problem for cross-compatibility between implementations. Enforcing the .mp4 = video rule via a whitelist of file extensions for each media type would resolve this.