Set the default applications on linux

Use xdg-mime to set default applications either for the whole system or the user account.

/usr/share/applications/defaults.list

~/.local/share/applications/defaults.list  //For an individual already has this file

Run apropos xdg and see some commands about xdg-series

Reference:

apropos - whatis データベースより文字列を検索する。
xdg-mime - command line tool for querying information about file type handling and adding descriptions for new file types

Mime types in Gnome/Ubuntu

Regarding adding new MIME types, apparently previous version of Gnome included "File Types and Programs" utility that did not make up into Gnome 2.8. New approach explained here
http://www.gnome.org/~jrb/files/mime/

For now on you can add new MIME types manually as described here
http://www.fedoraforum.org/forum/showthread.php?t=26875
http://www.au.freebsd.org/gnome/docs/faq2.html#q22
http://www.freedesktop.org/wiki/Standards_2fAddingMIMETutor

There is a graphical MIME editor that is related to ROX file manager (It is not included into rox-filer package, so you need to install it yourself)
http://rox.sourceforge.net/phpwiki/index.php/MIME-Editor