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 10.7.5. If you are not on version 10.7.5 it is recommended to upgrade.
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