Source: http://askubuntu.com/questions/369967/how-do-i-set-the-default-file-association-in-ubuntu-13-10-nautilus-files-with-un

One can use «mimeopen -d file» to invoke an application to open a file in the command line. Afterwards the used application will be the default application associated with the file type, even from the Nautilus.

Example: Set nano as the default program to open *.txt files:

$ 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)