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Probably all "slots" would be "occupied" and everybody gets the attacker's sent file, iiuc.

Or the attacker only needs to monitor the last free slot to see what shows up?

Seems to work both ways. Receive random file from unknown people or send chosen file to unknown people?

EDIT: was just explained in other comment by meejah, thanks! (https://news.ycombinator.com/item?id=14649727#14650551)