It looks as though there are many more types for mediatype:
Movie, Normal, Audiobook, Music Video, Short Film, TV Show, Ringtone
And: these categories are only valid for MP4 files anyway (which in a way has been indicated by you as you want to delete the field from mp3s)
You could create an action of the type "Format value" for MEDIATYPE
Format string: $ifgreater($strstr(%_tag%,MP4),0,Music Video,)
Thanks much ohrenkino. From what I can tell, that thread discussed how to do it manually, which I've been doing for a while now. Trying to automatically do it. I used your string suggestion (format value, mediatype, string $ifgreater($strstr(%_tag%,MP4),0,Music Video,) and it simply changes it to BLANK even on mp4's. Also tried the string $ifgreater($strstr(%_extension%,mp4),0,Music Video,). Just getting BLANK on all mp4's.