From <http://wiki.xiph.org/index.php/MIME_Types_and_File_Extensions>:
> .oga - audio/ogg
>
>    * Ogg Audio Profile (audio in Ogg container)
>    * Applications supporting .oga, .ogv SHOULD support decoding
>      from muxed Ogg streams
>    * Covers Ogg FLAC, Ghost, and OggPCM
>    * Although they share the same MIME type, Vorbis and Speex
>      use different file extensions.
>    * SHOULD contain a Skeleton logical bitstream.
>    * Vorbis and Speex may use .oga, but it is not the
>      prefered(sic) method of distributing these files because of
>      backwards-compatibility issues.

Thanks to Qball and Rasi for the patch.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7191 09075e82-0dd4-0310-85a5-a0d7c8717e4f