#setting Xorg xdg-open default application for file
#get file type of file 
xdg-mime query filetype image.gif

#get which desktop file that file type is set to use
xdg-mime query default image/gif

#list available desktop files
ls /usr/share/applications/*

#set new default application by using desktop file
xdg-mime default org.gnome.eog.desktop image/gif