Ah, I misinterperetted your post. I thought you meant to check it in Zimbra, because that's the only place where the "Show Original" option is present.
After checking the message source in Thunderbird you were right, the "Content-Disposition: inline" line is present. Did a quick Google search. Found that the solution was to edit the user's prefs.
js file and add the line 'user_pref("mail.content_disposition_type", 1);'.
Thanks a bunch for the help.
