Determine the audio format from a filename extension.
A filename or path whose extension is used for detection (e.g. "track.mp3" or "/music/song.flac").
"track.mp3"
"/music/song.flac"
A format key string (e.g. "mpeg", "flac"), or null when the extension is not recognised.
"mpeg"
"flac"
null
Determine the audio format from a filename extension.