They know who you are by identifying the IP address that their computer is sending the file to. That's right, their computer!! That's the only way to obtain absolute proof that someone is DLing the file, to be the one sending it!

Once they have that IP, they simply resolve the first sets of numbers into the service provider's network ID. Then, with the last numbers (along with the time and day), the ISP can identify the user. They can be subpoenaed or bullied into providing this service to a sufficiently influential industry.