GNOME 3 has dropped its capability to add custom command as the default application to open specific file with specific formats. In Gnome 2, we could do this by select at the file right click > properties > Custom Command. But, in Gnome 3, this feature is dropped.
And so, how to replace the default application to open specific file with our preferred favorite application. Example : we want to replace Ubuntu Software Center with Gdebi to open .deb files as default. The only way is : we must add our application manually, by editing application list file configuration named /etc/gnome/defaults.list or by creating a local mimetype list in our home folder named /home/ourname/.local/share/applications/mimeapps.list.
Both ways work, but the easier is by editing file /etc/gnome/defaults.list. To edit this file, just simply type in terminal : sudo gedit /etc/gnome/defaults.list. After opened, we will see long lines about default mimetypes with their default applications to open.
As an example, we want to replace Ubuntu Software Center with Gdebi, these are the steps :
The most first, of course, we have to install gdebi by apt : sudo apt-get install gdebi
Find two lines and replace them
with these :
logout and login back.
If we want to replace another default applications (such as : evince with foxit-reader to open pdf), just do the same way. Find the file type, and replace its defaults application to open the file.