I've seen a few tutorials of how to use Html5 File API such as this https://www.html5rocks.com/en/tutorials/file/dndfiles/ for file uploading. What I want is a few pointers of how to send all that from a browser to backend and process it there to actually upload files. In those tutorials it's not shown.
The backend language doesn't matter.
And I don't want to use an external js library, only pure javascript.