Need help with mime types & file associations

[Joshua Smith <jesmith kaon com>]

Nautilus Guri,

First, the caveats: I REALLY REALLY tried to figure this out myself, 
and found lots of thoroughly unhelpful hints on the web.  I tried to 
look in the Gnome FAQ Wiki, but the web server said "permission 
denied", and I tried to search the archive of this mailing list, but 
that is broken too.  If there is a better place to post this question, 
please let me know.

Problem Summary:

I am trying to configure an installer for my software, such that when 
the user tries to open a file with a particular extension, my program 
runs and is given that file as it's argument.  (On Windows, I do this 
through a bunch of cryptic registry entries; on the Mac, I put some 
voodoo into an Info.plist file.)

Details:

1. My file format is really ZIP/JAR with a different extension, just 
like OpenOffice docs.
2. I need this to work with Sun JDS, which uses some version of Gnome 
and version 2.6.0 of Nautilus.  (I don't know how to find out what 
version of Gnome is running.)
3. The desktop is ABSOLUTELY CONVINCED that my files are mime type 
application/zip
4. I'm very comfortable with Unix, dating back decades, but I have no 
experience whatsoever with modern Unix GUIs

What I think I know:

A) I need to convince the desktop that files ending in .mesonjar are 
mime type application/x-meson
B) I need to convince the desktop to open application/x-meson files 
using my program

I'm mostly focused on A, since B seems to be pretty well documented 
(.desktop files seem to be the key), and I have to get A done before I 
can start to think about getting B right.

I read the man page on mime magic, which says that file sniffing is 
going to take priority over anything else I do.  That looked bad until 
I looked at the magic file, and found the line for ZIP is commented out 
because it would keep StarOffice from recognizing its own files 
(according to a comment).  This does leave me quite mystified as to how 
it knows my files are really ZIPs... is there something else that does 
the same thing as /etc/gnome-vfs-mime-magic?

There is a lot of conflicting documentation about how to put more mime 
types into the system, but nothing I try seems to have any effect.  I 
think the system may have cached its conclusion that my .mesonjar files 
are ZIPs someplace, but I cannot figure out how to clear/refresh that 
cache.  (I found documentation of a command called 
"update-desktop-database" but there is no such program on my test 
system.)

My concise question:

How do I convince my system that files ending in .mesonjar are mime 
type application/x-meson, even though they are really just renamed ZIP 
files?

-Joshua