File "types" in gnome

[Jonathan Ryshpan]

How does Gnome determine the "type" of a file.  The type can be
discovered by right clicking on the file icon in Nautilus, and then
selecting the tab "Open With".  I suspect that the type is determined by
the type as reported by the file command, by the file's extension, and
by tables of mime types in the system.  Is this correct?  The rules
don't seem to be documented anywhere, nor is there any advice on how to
set up the tables.

Why am I interested?

I'm trying to set up nautilus so that clicking on a XML file with
extension ".aup" will start Audacity.  (".aup" is the extension used by
the Audacity audio editor for the files it uses.)  On my system, the
type of such an .aup file is "XML document", which is associated (quite
reasonably) with Firefox and a number of editors.  I'd like to set
things up so that files with extension .aup will have their own type,
which could be associated with Audacity.  How can such a type be
created, etc.?

Thanks - jon