How can I make attachements downloadable only for registered users?
I think that this is standard. The attachment quota for guest is default equal to 0. See AdminCP->Users&Groups->Guests->Forum&Posts attachment quota should be set to 0.
Almost.
It's "Can download attachments?" checkbox. It's by default unchecked.
It also can be found in AdminCP->Users&Groups->Guests->Forum&Post
When attachement quota is set to 0 then it means no-limit.
You are right, I looked in my setting and there it was 0 because I do not allow guests
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