I reckon "the first byte you upload" is being used as a turn-of-phrase here, and is not meant not to refer to the specific bit-sequence of each (or the first) b

I reckon "the first byte you upload" is being used as a turn-of-phrase here, and is not meant not to refer to the specific bit-sequence of each (or the first) byte of the upload. Rather the idea that the (eventual) metadata of the whole upload, (which I suppose actually begins before the first byte of the payload), is what would trigger the letter.