While I really don't know much about mimetype naming, this builds without issue. A test .cbr file opened fine both with and without this, so not sure what the best test of this is.

I'm sorry, I should have added more context. The main issue from my perspective is that thumbnails aren't being generated for .cbr files unless I manually edit /usr/share/thumbnailers/atril.thumbnailer. The correct mimetype for these files can already be found in backend/comics/comicsdocument.atril-backend.desktop.in. Here's the official doc: https://www.iana.org/assignments/media-types/application/vnd.comicbook-rar

In my case I don't get a thumbnail with or without the applied, but I am not working with anything near a default install so that may be impossible to diagnose