Pay Per download.
Basically a user uploads a file to your servers/sites and it is locked by a survey of which the person wanting to download it has to complete.
These surveys pay anything from 50c to $20 for certain countries.
if you have a big user base after mass advertisements and showing legitimacy you will have hundreds of users using hundreds of files.
They get the payout of the survey - your % you charge which is normally about 5% so if someone uploads a file and someone downloads in france and has to use their phone it normally makes like $15 so 5% or 15 is 75cents made for free now x that buy y and you've made your investment back in the first month.