Actually, was caused by down track not initialising mime, but it is good
to ignore unknown mime.
Also, added ulpfec sa SDP has that and there was a conversion which was
returning unknown. Also, note that the mime types use audio for RED and
video for flexfec and ulpfec although they are not media type dependent.
Maybe, at point need to introduce `MimeTypeAudioRED` and
`MimeTypeVideoRED`.