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A length extension attack doesn't let you find a collision. It lets you predict the hash for an input with an unknown component in the prefix. If you have $h = H(x)$ for unknown (or partially unknown) $x$, you can generate $h_y = H(x \vert\vert y)$ for arbitrary $y$ (this is not strictly correct; I've ignored padding, but for the purposes of this discussion ...


This answers a comment to Stephen Touset's fine answer. With SHA-256, or any collision-resistant hash, no known attack (including length extension) allows producing a file different from the original file and that has the same hash as the original, even if an adversary could choose the original. Even with the practically-broken MD5, or the broken SHA-1, no ...

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