Require Paid Subscription to Download Attatchment?
 

Is there any way to do this, beit a mod or otherwise?

If you are referring to attachments in general, simply change all existing groups to not be able to download attachments, then add a new usergroup for the subscription that can download attachments.

If you're talking about just doing it for one attachment, it's possible with the above system by making a designated forum for said attachment and only allowing the subscription usergroup to download attachments from said forum. That may or may not be overkill, though.

I don't know of any modifications that would do this for single attachments.