Use Readable instead of ReadStream in uploadV2

[redneb]

Summary

The old upload method accepts a Readable for its file argument, but the new uploadV2 accepts a ReadStream. There is no reason to use ReadStream here: Readable is strictly more generic (it's a superclass of ReadStream) and the code actually does not use any functionality specific to ReadStream, it only uses methods and events from Readable. This will make the transition from upload to uploadV2 more easy.

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[CLAassistant]

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