So, we're not commenting on "That hash is guaranteed by cryptography to always only represent the contents of that file"? Because we should be. The fundamental rule of hashing is that you are guaranteed to have collisions. The challenge is to find a good hashing function for the kind of file you're hashing, but on the internet you get EVERY TYPE OF FILE, so at the scale we're talking about for IPFS, we're dealing with a large enough number of files of every conceivable type that collisions are guaranteed.
You're thinking of non-cryptographic hashes. A cryptographic hash that could generate collisions in any practical situation would be considered broken. You'd need more files than there are atoms in the galaxy to have even a 1 in a billion chance of collision with a secure 512 bit hash.
