They don't block uploads, they monitor sites like this one, and report any links to the upload host site. In reality they only need to monitor a handful of sites as once something leaks the file is shared around.
I uploaded a password protected zipped up blank word document with a completely random name and pasted a link to it on f95 pretending that it was a leaked Fansadox comic. it was taken down within hours. This was the only place it was posted. It got taken down despite not containing anything other than random blank zip file.