Actually, I just decided to try out the VLC backend again to see if changing mimetypes would indeed work, and apparently the MIDI issues with the backend have been fixed while I was not looking! That's pretty awesome! It now recognises MIDI files and can play them back, just like GStreamer. The difference being that VLC is using FluidSynth and GStreamer is using Timidity. And I actually like FluidSynth a bit more... Now if only Amarok was to add MIDI files to the music collection.