The problem is probably in the users' mimeTypes.rdf file. If so, consider voting for
http://bugzilla.zimbra.com/show_bug.cgi?id=32536
How the problem usually starts: your users download a file from a web server with a bogus mime.types file. Firefox stores the mime type in the user's profile. User goes to upload a file with the same extension to zimbra webmail. Firefox tags it with the (bad) saved mime type, which the Zimbra server doesn't validate.