How do I set the default file association in Ubuntu Nautilus to something that is not listed in the Open With tab?

Solution 1:

one can use mimeopen -d file to use a custom application on the cli. Afterwards it will be the default application [even from the Nautilus].

$ mimeopen -d file.txt

Please choose a default application for files of type text/plain

      1) notepad  (wine-extension-txt)
      2) gedit  (gedit)
      3) GNU Emacs 23  (emacs23)
      4) Other...

use application # 4

use command: nano

Opening "useful.txt" with nano (text/plain)

There are other ways you can find here