There are so many mime types that often the authors of icon themes do not make icons to every mime type. It cause that many MIMEs (often rare) don't have its icon. Many of them are audio, video or picture formats, sometimes documents. The problem exists especially when an application adds new mimes, which are not included in standard mime types from freedesktop.org. Obviously the application can add icons to new mime, but the new icons often don't match to using icon theme.

In my opinion good solution is to add field to scecify another icon if the dedicated is unavailable to shared-mime-info-spec. Also some generic icons may be added to icon-naming-spec (image-generic, audio-generic etc) to let new field point to icons which surely exist, but it's the problem for another report.

Egzample:
Program TuxGuitar (guitar tabulature editor) adds mime type audio/x-ptb, audio/x-gtp and audio/x-tuxguitar. Any icon theme does not contain icons to this file types. All of these mimes contain data similar to midi files so the best solution is using icons of audio/midi mime. If current icon theme doesn't contain icon for audio/midi, the audio/x-ptb would use audio-generic icon.