This has been fixed in IMP 5.

Un-stalling for IMP 5.

Okay, so I'm stalling (essentially resolving) this until such time as 
we have a new MIME lib (assuming IMP 5/Horde 4, though I'd really like 
to see it possible to bundle an updated whole MIME package with an IMP 
4 version instead, but your call), and have added notes to the wiki on 
what a new MIME lib should have, quoting you:

http://wiki.horde.org/NewMIMELib

What I would eventually like is to refer to these embedded mime parts 
by an id.  PGP Example:
0. multipart/signed
   1. encryption signature/version stuff
   2. encryped data
     Encrypted data contains:
     1. multipart/mixed
       1. text/plain
       2. image/jpeg

Then, if we want to access the image/jpeg option, we do a 
getPart('2.2') call to the MIME_Contents:: object and it will be 
responsible for rebuilding the data (it sees that 2.2 doesn't exist on 
the IMAP server, but that id 2 can contain embedded data, so it 
automatically downloads/parses as many parent parts as required).   
This will eliminate the need for MIME specific caching - which was 
always kind of a hackish implementation by me I admit.  But, alas, I 
think this will require enough changes that it is just not worth it to 
do the coding gymnastics necessary to cram everything into IMP for 
purposes maintaining BC.

Michael - I'd like your assessment on whether it's worth including a 
revamped MIME_Contents package in IMP that can handle this sort of 
thing, or if this ticket should be closed as undoable?

Unfortunately, this is the same exact issue that prevents us from 
being able to quote signed/encrypted text in replies/forwards.  I'm 
not sure how much we can do without terribly breaking BC (or, at the 
least, rewriting MIME_Contents:: in IMP just to work around BC issues).

Okay, the file isn't going to attach, so you'll just have to do 
without an example for now. :P

Okay, and the file...

Instead, I get just the signature file. Attached is a (zipped, large) 
message that demonstrates the problem. Also, not sure if it should or 
not, but in the inlined attachment listings for the images, there's no 
Add To Gallery option.