Yeah, the bug in question was marked as a duplicate of bug 6736, fixed in 4.5.4. You can check bugzilla to see the details.
I am using an Outlook 2010 trial. I am sending to an IMAP account on a laptop running Zimbra. The standard email (html) is sent without an attachment but arrives with Unknown <text/rtf> in Zimbra and in MAC Mail being used by a client. He says he now has a desktop image he can't remove. It is 0kb but has a link to Zimbra offers Open Source email server software and shared calendar for Linux and the Mac. which I hesitate to pursue. I have yet to see the image.
I suspect this may not fundamentally be a Zimbra problem - but I'm out my depth so can't say more. Could be an Outlook bug?
Ah! The link I included above has been replaced by a URL on the Zimbra server which returns a page with Unauthorised Access. The link I originally posted - without the http:// is "127.0.0.1:7633"
So what's going on guys ;-)