This concept is often applied to URL filtering i.e. I want this user to get to that site, but that user to get to only those sites...
Its discussed at length in this article:
The concept is to define the groups/users/IPs/sites that you want to allow uploads, and set a flag which will allow or block them later in the rules.
I have done creating the web mapping (policy assignment) but im a bit lost on how do I set the event to allow certain users/groups to upload / download media filtering by certain website with specific file extension .doc / .docx. Do you have any tutorial for this?