Change default application for xdg-open

NILI write it as a note for me and if it can helps some other people,

it's fine.

To change the program used by **xdg-open** for opening some kind of

file, it's not that hard.

First, check the type of the file:

$ xdg-mime query filetype file.pdf

application/pdf

Then, choose the right tool for handling this type:

$ xdg-mime default mupdf.desktop application/pdf

Honestly, having firefox opening PDF files with GIMP **IS NOT FUN**.