It looks like, again, issues with UTF-8 encoding in SmarterMail, affecting also the attachment filenames. The same encoding issues existed in webmail with subjects, some headers, etc... and were fixed recently. Are the mails with this issue composed from the webmail ? I guess the issue will not exist if you use a mail client to send your mails. It is really important that SmarterTools review and fix these encoding issues everywhere it's used and once for good and not a little bit here and there.
I can't reproduce this issue with build 7699. Can I ask which build you are using ? Maybe it's a build not containing the recent UTF-8 issues fixes.