How to set a default application to open a file in mac?
I don't know why the existing two methods are not working - they both should.

Alternatively try RCDefaultApp which as far as I'm aware still works in Catalina. [I can only test up to Mojave.]

I added this manually using the + button &amp; it actually suggested TextEdit itself. I'd suggest also checking 'Override file creator' &amp; see if that might help.