how about a random question based on politics.
you should be able to guess the users reaction based on the file they wish to download.
for example if you are downloading h mangos, asked the question: are you for or against freedom of speech.
most humans would awnser "Yes" as more people awnser, then a profile of what people downloading that file would think is built up.
that is how you can check whether or not the person is a bot.
of course initially, each response would have to be accepted, so it would only work after the first 100 or so replies.
a bot wouldnt know how to awnser such questions.