> US-centric here: I feel that uploading a small percentage of a file as a condition of downloading the whole thing may very well fall under fair use

This is not consistent with current Fair Use application. The TL;DR is that the use must be for a new expression that is protected by either the 1st amendment or copyright itself. I don't know of a single case where it has applied to mere distribution. I would be astounded if there is such a case because that isn't within the expressed purpose of the doctrine.