Who pays for the files on Skynet?
There's a couple situations we want to consider here.
If you upload the file from your own Sia node...
You pay for files you upload to Skynet. You can use the command:
siac skynet upload [localpath] [siapath]
the file is being uploaded to your own contract set and will stay online as long as you continue to pay for the file.
If you pin someone else's file...
The same is true if you pin a file someone else uploaded to Skynet. Using the command
siac skynet pin [skylink] [siapath]
will ensure the file stays online using your contracts.
If you upload the file to a webportal...
There are a number of webportals currently operating on Skynet. You can find a list on the bottom of our official site (which is also a Skynet webportal).
The webportal operator pays for the file, and may choose to monetize their site a number of ways. Examples might include longer pinning, ads on the website, ads injected on videos/audios being downloaded, paying for extra bandwidth and latency. It's up to the portal operator how they want to monetize the service.
When you download a file...
The node that pinned or uploaded a file isn't necessarily the one who pays for the download costs. If you download from your own Sia node, you pay for the bandwidth. If you download from a webportal, the webportal pays, and again will monetize however they see fit.