Symantec Messaging Gateway (SMG) does not match attachment file names with entries in a custom Content Filtering Dictionary when the attachment file name contain a left square bracket '['.
This issue has been resolved in the SMG 10.8.1 release. If you are not on version 10.8.1 or later, Broadcom recommends that the Messaging Gateway be updated to the latest release.
Workaround for older versions
Rather than use a dictionary match for file names with a left square bracket, use the has a file name which contains match