[Nautilus-list] How do I add a new mime type for nautilus

[ottffss <ottffss worldnet att net>]

I've tried adding a new mime type to my installation of nautilus
(application/mathematica) and nothing seems to happen.  I've also added
the entry to the file /usr/share/mime-info/gnome.mime (as root) and
nothing seems to happen either (i.e., double clicking a file such a s
test.nb doesn't launch mathematica).  I've also searched my file system
for a file named gnome-vfs.mime since the header in
/usr/share/mime-info/gnome.mime  says that is where nautilus will
(might) look for mime types.

I've also tried to change the icon used to represent .nb files both
using the gnome control panel and by editing the file
/usr/share/mime-info/mc.keys.  Again, no effect.

Is this a bug of am I doing something stupid.  If there is a work-around
I would like to try it (maybe I've edited the wrong files) since I
really like the look of nautilus.

PS
what is the sidebar loser supposed to do?