Access for attachments only for specific usergroup

[4yBak]

Hello. In specific forum I want grant access download attachments only for users from local network (I create new usergroup and move local user here).

Why this do? We have slow internet connections and we want grant access for download attachments in specific forum for local users, and for other users give link in internet resource.

Example (something like this):
1) link for local users - see attachments
2) other users - http://bla-bla-bla.com/1.zip

How I can do it? May be already exist modification for my problem?

Note: I use vB 3.6.0