MyBB 1.6* and 1.8*
Redirects "non-premium" users to a subscription/upgrade page whenever they try to download attachments that are for premium users.
Redirect page, premium usergroup(s), forum(s) and redirect messages are all configurable.
This does not include an upgrade/subscription page. This will only redirect (non-premium-)users to that page, when they are trying to download the premium items.
Sorry, but you need at least 25 posts and 5 hours online time to unlock hidden content.