The attack has nothing to do with hash collisions; it has to do with the fact that the MD-structured hashes spit out their entire state at the end of the operation, which means an attacker can simply reformat the hash back into the hash core's state and continue hashing with it.
I didn't think so, but I mentioned it because the Wikipedia article on Merkle-Damgård hashes (http://en.wikipedia.org/wiki/Merkle–Damgård_construction) talks about length extension attacks only in the context of hash collisions: "Length extension — once an attacker has one collision, he can find more very cheaply."
Multi-collisions is another thing. What Wikipedia means is that once you find m and m' s.t. H(m) = H(m'), then you've also found the collision H(m || X) = H(m' || X) for whatever X, i.e., unlimited collisions.
