upload: support files over 2^31 bytes

[grahamc]

Description of changes:

Multiplying offset and blocks as i32 broke on larger images.

Co-authored-by: Cole Helbling cole.helbling@determinate.systems

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It would be really better if the interface didn't take signed integers at all, I assume this is Java internals leaking out through the generator.

[grahamc]

(rebased)

[cbgbt]

This looks great, thanks very much!

[grahamc]

Thanks!