All Barracuda Spam & Virus Firewalls, firmware versions 3.5.11 and above.
In 3.5.11 and above, if any attachment filtering rules exist for any substring of the word empty, like mp or pty, the Barracuda Spam & Virus Firewall will block said emails if they contain empty attachments. This can be avoided if the attachment filters in question have the . in front of the extensions listed, like .mp or .pty.
If the Barracuda Spam & Virus Firewall is blocking emails for this reason, the headers of the email (viewable via the Basic > Message Log page) will contain something like this:
X-Barracuda-Filename-Alert: BANNED FILENAME, message contains part named:
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