Gnome splits the settings for default application in system-wide
and user specific settings. System wide application assignments can be
found in /etc/gnome/defaults.list. User specific
files are located in ~/.local/share/applications.
If you make changes to this files execute 'update-desktop-database' as
user root to apply changes.
Using the file browser Nautilus makes the process easy:
Select a file and choose Properties.
Go to tab "Open with"
Choose a gnome suggested program from list or add a new one by using the Add button.
The Program, whose radio box is marked, will be the default application for that file type.