from http://bugs.musicbrainz.org/ticket/5259

When the tagger encounters a file with tags in a format it isn't expecting (presumably checking by extension) it simply refuses to read it. Or alternatively, if it encounters a file extension it doesn't recognise, it won't try anyway. An example is the ".aac" HE-AAC files produced by Winamp, which use ID3 (v2.4 I think) tags. With the ".aac" extension, it won't open them. If I rename them to ".m4a" or similar, it will read them, and display the tags, but won't do anything with them, citing a "moov not found" error.

But if I simply change the extension to ".mp3" it reads and tags them with no issues whatsoever. It'd be great if when the tagger can't find tags it's expecting, it just did a "brute force" search for ANY tags it can handle. I'm using Windows 7 x64, but this issue also occurs on Windows Vista x86, and Linux AMD64.

outsidecontext replied with

As a side note Picard already supports some simple mime type detection for image files. This could be expanded for audio files, too.

and luks with

Mutagen itself supports that, see the score method on file format classes. That will return a score based on scanning the first few bytes of the file and the extension.