document how to get the data of a file downloaded with "filesDownload(args)"

**Why is this feature valuable to you? Does it solve a problem you're having?**
It's hard to tell how to do that, first i could not have known if that was even possible if i haven't looked at the HTTP Api documentation, i have to poke around the source code (which toke a while), to figure out what was happening.

**Describe the solution you'd like**
document how to get the file data.

**Describe alternatives you've considered**
document that the javascript API documentation is incomplete, instead have a link to the HTTP Api docs which seems much better.