it depends. If lots of people download your files or your videos then answer is yes, why not?. I did this for a while, but I didn't earn much. However, they reward little and that's right. But it is not bad as an extra income
though i concede that there are better ways to make money but doing paid to post can give you a substantial income to help add to whatever you are earning at the moment.
when you paid some one to develop a site for you it we help you to make more money,when you advertise things there ,the more people views it you are making money, expecially your skills and your product in times of business.
It depends on what kind of sites you are using, what kind of files you are required to upload, and how much they pay for uploading. BY the way, these sites are not my favorite kind of earning site.