It should be added of course. There are a couple of challenges:
For Files gallery to know this is a video file, the file needs mime type
*.avif can also be an image format
image/avif so we can't assume anything from the extension alone. We could use server/PHP to get mime type for
avif files, as it will know the difference between image and video, but then you need an updated server that outputs correct mime types for the
avif format ...
FFmpeg .avif support
Files uses FFmpeg to create thumbnails, and support for avif was added only a few months ago, so you would need a recent version of FFmpeg installed on your server.
Do you happen to have any smallish avif video sample file lying around? I can't find anything on the internet.