avformat/matroskaenc: Improve mimetype search

avformat/matroskaenc: Improve mimetype search

Use the mime_types of the corresponding AVCodecDescriptor instead of
tables specific to Matroska. The former are generally more encompassing:
They contain every item of the current lists except "text/plain" for
AV_CODEC_ID_TEXT and "binary" for AV_CODEC_ID_BIN_DATA.

The former has been preserved by special-casing it while the latter is
a hack added in c9212abf so that the demuxer (which uses the same tables)
sets the appropriate CodecID for broken files ("binary" is not a correct
mime type at all); using it for the muxer was a mistake. The correct
mime type for AV_CODEC_ID_BIN_DATA is "application/octet-stream" and
this is what one gets from the AVCodecDescriptor.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>