I think fopnu applies some basic rules for distinguising files (example: .mp3 -> audio), but it's missing numerous audio files. For example: ".xm" files are audio and you can't find them unless you select "Other". Same happens with these other audio files: .mod, .s3m, .mo3, .dsd, .it, .opus, .wma, .mid, .midi, .ape or .v2m. I am sure other rare formats are also unsupported, but I am surprised that opus or ape aren't caught properly by Fopnu and are somewhat popular.

I think the same happens to other file categories such as video, images and text. For example .webp doesn't show up with Images.

Bonus suggestion: When using the search bar, you shouldn't get your own files as result when you search.

I agree as well, but I don't think it should be "hard-wired" into fopnu. Rather I think that fopnu should have the same categories that it does now, but the user has the ability to add/remove/toggle/edit the associated extensions for each category.

If I want to search for an audio file, but do not want any lossy formats then I'd select audio & uncheck all audio formats that aren't lossless.