Hi,
This problem is specific to Zettlr, but I think the cause may be more system-level. I’ll ask this question also with Zettlr and report back.
Normally, you can open and edit tex-files with Zettlr. My system doesn’t show Zettlr as application to open tex-files with. Even a tex-file made with Zettlr, is not visible if you choose open from Zettlr’s menu and it’s not in the recent files menu.
I’m using the Flathub version, maybe I could try the AppImage.
I only need Zettlr to recognise the file type, because I have my own export profile. There you have to point to the tex file, but it’s not recognised there either.
EDIT:
With the app-image, you can open a tex-file with Zettlr, it shows in the menu. It also shows when you choose “open”, but there it’s greyed out.