All Email Security Gateway, 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 Email Security Gateway 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 Email Security Gateway 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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