I've had to think about MIME types recently. I'm surprised that I can't find any definitive list of MIME types. The list at IANA seems to be the most official but it lacks very commonly-used stuff (such as audio/midi). I feel untethered when standards are only de-facto.
It looks like the draft Freedesktop shared MIME database will help. Apparently GTK+ 2.4 is already using it. I had no idea that this had come so far.