I am not shown the "Open with ..." tab in Files or the "Set as default ..." button anymore. This seems to be gone due to a UI change and there is an issue registered at Launchpad.
I know that tools like Ubuntu Tweak have offered such functionality in the past (it is mentioned as a solution in similar questions) but afaik Ubuntu Tweak is no more supported (i.e. not in the repo). One can use
mimeopen to adjust this from the command line. And, there is a relatively complicated but generic workaround.
However, I am looking for a GUI-based way and without installing extra packages not in the default Ubuntu repos. Thanks for help!