Editing mime-type list?

[HpG <gisler ntb ch>]

Hi all

I got a question regarding mime-types of GNOME.
How can I add mime-types by my self?
The reason for this (perhaps silly) question
is, that I am working with Lyx which creates
*.lyx files. I can not find that extension
in the mime-type list.
I whould like to open automatically Lyx
when double clicking in gmc on such a file.

Thanks for any help
Hanspeter