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Considering it may have been binary, and binary would make up the majority of it (images, sounds, video are large), I'm guessing 0x00.

Assuming a uniform distribution, there's only a 1/256 chance of that.

images, sounds, video are large

these are unlikely to be a uniform distribution though

These are usually compressed, so likely to be a uniform distribution.

The remainder isn't. There are always headers and such.

If they weren't, they could be compressed better.