[Wylug-help] setting the default application for a particular file mime type in gnome.

[Linda Kennedy & David Carpenter]

Had a quick look around, and came up with this advice at the gnome site.

I think you have to first create the new MIME type on the system (the 
xxx in file.xxx)

Have a look at Modifying MIME types:
http://www.gnome.org/learn/admin-guide/2.14/mimetypes-modifying.html
There is an example on how to do it here:
http://www.gnome.org/learn/admin-guide/2.14/mimetypes-modifying.html#mimetypes-newtype-example

I think once you have done this then the 'right click' suggestions 
others have suggested and you got stuck with because the MIME type was 
automatically picked up as text/plain (as described in the how to...) 
may well work, if not.....


Maybe you want to look at:
Registering Applications for MIME Types - 
http://www.gnome.org/learn/admin-guide/2.14/mimetypes-registering.html

Notes:
I ran a search on .desktop on my file system to find the .desktop files 
this article speaks of, and opened one up in gedit and it seemed to make 
some sense. i.e. there was a MIME type entry where I could add another 
mime type, and it was a file that had TYPE = Application.

The page I've referenced says that it's straight forward, but it's not 
something I've done.

The instructions are for Gnome 2.14 I believe.

There is a whole section here on mime types if you check the links on 
the right hand side of the page.

Hope this is of some use.

David