Symantec Messaging Gateway (SMG) identifies Visio .vsdx files as archives rather than as Microsoft Office 2013 documents.
This issue has been addressed with the SMG 10.6.3 release.
How to detect .vsdx files as Microsoft Office 2013 document:
Policies can be written to identify Visio (.vsdx) documents by creating an attachment list based on the .vsdx attatchment via the Content -> Resources -> Attachment Lists page and then implementing that list with the following policy logic:
If the attachment or body part is in the attachment list "Archive Files (default)"
If the attachment or body part is in the attachment list "Visio Files"
Where "Visio Files" is the name of the attachment list created for the .vsdx file extension.