libaformat/mp3dec: Register the MIME type "audio/mpeg" to the mp3 decoder.

Some mp3 files look surprisingly like mpeg PS streams. In order for us
to hint to ffmpeg that the file really is mp3, we can deliver it with a
mime type. The mp3 probe therefore needs to have a mime type registered,
(which is previously did not).

libavformat/mp3dec.c

@@ -613,6 +613,7 @@ const FFInputFormat ff_mp3_demuxer = {
     .p.flags        = AVFMT_GENERIC_INDEX,
     .p.extensions   = "mp2,mp3,m2a,mpa", /* XXX: use probe */
     .p.priv_class   = &demuxer_class,
+    .p.mime_type    = "audio/mpeg",
     .read_probe     = mp3_read_probe,
     .read_header    = mp3_read_header,
     .read_packet    = mp3_read_packet,