Still should have to do with RFC-2047 vs RFC-2231. As explained here from thunderbird: http://kb.mozillazine.org/Attachments_renamed
Exchange 2007 (and up) have support for RFC-2231:
So Exchange 2003 (and down) don't, and wouldn't be able to handle the newer RFC-2231 formatting, which was introduced and defaulted in newer vmime version.
This change happened in libvmime-0.9.2+svn603-2, which should have fixed the bug and was so reported on the forum: https://forums.zarafa.com/showthread.php?7083-solved-Attachments-received-as-ATTxxxxx-dat