There's nothing exactly like this.
http://bugzilla.zimbra.com/show_bug.cgi?id=4882 is the closest, a postscript attachment is not recognized in WebUI. But it's still different: my message displays in the UI with attachment links, whereas 4882's message doesn't even show the attachment links. While we have the same problem of the attachments not showing up inline, it seems the nature of the problem is different.
Anyway, I'm starting to think this may be a problem in the creation of this message - I've been able to display attached inline images from other messages. I still really need to figure out why these messages don't display correctly, they are ones we send to all our customers. Do you have any suggestions for me to verify message correctness, as zimbra parses it?
Anyway, this really may not be a big deal, I've only seen the problem in the Zimbra webUI. But it's worrisome.