RFC 3261 standardized the MIME types for wave audio files and AVI video files as audio/vnd.wave and video/vnd.avi respectively. However, in Gentoo /etc/mime.types, the .wav and .avi file extensions are only associated with audio/x-wav and video/x-msvideo respectively.
Oops, I typoed. I meant RFC 2361, not 3261. I've made a new bug and am closing this one (since I can't seem to edit the title).