Today there is more then 19.000 different kinds of file extensions in use in the world. I would be nice for security to have a white-list instead of a blacklist attachment filter. The attacker allways decide witch kind of extension to use ore misuse in the attacks.
So if i only want to allow .DOCX then the 18.999 is by default denied.
This wil olso help to ensure the the AV scanning engine really can do a nicer scanning job and improve security a lot.
Francis Van den Eynde commented
i think this is already possible.
you could make 2 new rules and place them at the top of the attachment filter.
Rule 1 : attachment .docx - action : accept
Rule 2 : attachment * - action : block
then only .docx would pass