To set file associations, there are 2 configurations you should care about:

• mime-type definition
• application association

To set mime-type definition, open /usr/share/mime/packages, and view the .xml files in it. You’ll find how to write your own mime-type definitions.

To set application associations, open /usr/share/applications, and view the .desktop files in it. You’ll find how to write your own .desktop files. Also, don’t forget to view the defaults.list file, which sets the default applications for certain mime-types.

There are also local settings, located at ~/.local/share/mime/packages and ~/.local/share/applications. Local settings will take precedence over global, i.e., system settings, by convention.

Finally, there is an example:

http://wiki.archlinux.org/index.php/Custom_File_Associations

Note. There are also tools to manage these tasks, such as xdg-mime.