For future reference:
To find out the 'mime type" of a file, if you don't know it yourself ( in this case a png ):
Code:
xdg-mime query filetype /tmp/a_png_file.png
This would return "image/png"
To find which application is currently the default for 'xdg-open' operations (aka, usually "most" things like double clicking stuff in a file manage)
For pngs, for example:
Code:
xdg-mime query default image/png
To set a default application:
In this case, open pngs with Krita
Code:
xdg-mime default krita.desktop image/png
Note: It might need a reboot, or at least a session restart (log out completely) to show the changes.
There is of course a better way for that but let's keep it like this for now, if you're a user who doesn't really care about keeping the computer up at all times.