XDG (freedesktop.org) which stands for X Desktop Group is a group which develop the X11 and xdg utilities which currently runs as barebones of linux desktop. So how do we change the defaults when opening a file on XServer?

Be free, and live life fully.

— Caroline Shaw.

So how do we configure XDG?

Tools that we will be using to configure xdg can be found in xdg-utils package (utilities like xdg-settings and xdg-mime).
So to set the default application to open a certain mime type, you need run the code below:

xdg-mime default Thunar.desktop inode/directory

The command above would add the new mime application settings to ~/.local/share/applications/defaults.list. It basically tells xdg-mime to open a directory using Thunar (XFCE filemanager).

Fun Fact

1. freedesktop.org (fd.o) is a project to work on interoperability and shared base technology for free-software desktop environments for the X Window System (X11) and Wayland on Linux and other Unix-like operating systems. It was founded by Havoc Pennington, a GNOME developer working for Red Hat in March 2000. The project’s servers are hosted by Portland State University, sponsored by Hewlett-Packard, Intel, and Google. ↩︎