Also hook to audio/mpeg MIME type

Some mp3 files are detected to be audio/mpeg instead of
audio/mp3. This seems to depend on the encoder used.

Example audio/mp3: http://wikitranslate.org/wiki/File:Разговор_о_шляпе_и_панаме_Аня.mp3
Example audio/mpeg: http://wikitranslate.org/wiki/File:Слон_-_Агния_Барто.mp3

(Untested.)