It's just soldered on. Normally there isn't a lot of force involved, and the soldered area is fairly large, so that is plenty strong. When my 3A got wrecked, it ripped one of the brackets off the inner fender, but didn't bother the radiator at all.
Kind of tricky to resolder that without getting one of the other joints too hot, IMO. If possible, I'd take it to a radiator shop.
BTW, part of the bracket is joined to the radiator frame, which is steel. The other side joins to the brass upper tank.