When you download a file from BOST, notice that it doesn't tell you the final file size. That's because you're downloading the output of a program, not a "file".
So it's pretty obvious that whatever program they're running when you download a file is putting "notes" in some place(s) in the fie, incl. the username of the BOST user who did the download.
In other words: pay for your media and don't share it to everyone, simple.