Yes, that was the first thing I looked at. It isn't making any difference what I do in the video section, because the WMV files are being treated as though they are music files.
It's not a big problem; I'm working around it by tagging up my video files in MusicBee. I'd rather not be doing that, as the tags seem to be a bit fragile (renaming the file in Windows Explorer loses the tags, so I suspect MusicBee is storing externally in the library rather than embedding them in the WMV file), but it's a bit irritating.
Thanks for your suggestions.