How does MIME handling exactly work?

[Gérard Milmeister <gemi bluewin ch>]

What is the exact procedure that gnome-vfs2 (as in nautilus) uses to
determine the mime-type?

Apparently the mime types in /usr/share/mime-info (and in
~.gnome/mime-info) are no longer used. I noticed this, when I tried to
create a mime type "application/mathematica" with extension "nb" for
Mathematica notebooks. This shows up in gnome-file-types-properties, but
other otherwise is completely ignored and gnomevfs-info
file://$PWD/measure.nb simply shows:
Name              : measure.nb
Type              : Regular
MIME type         : text/plain
So, is it true, that a user can assign an application to a mime type
that already exists, but not create new ones?

-- 
Gérard Milmeister
Tannenrauchstrasse 35
8038 Zürich
gemi bluewin ch