Download permission

[Jackie1]

I see a forum like this use

if your Group are not equal or higher than Member Group, You are not allowed to download attachments.

To be Member Group,you need:
1.at least 1 posts;
2.registered more than 1 days;
3.your reputation is equal or higher than 12. (From May 1,2009)

And
after Jul 1, 2009, you need reputation >13
after Sep 1, 2009, you need reputation >14
after Nov 1, 2009, you need reputation >15

Can tell me they used mod or how to do so?
Thanks