We have some support for async upload by url. It was disabled because people wanted to wait for echo, so we could have reasonable user notifications.
Users want to be able to upload files that are up to 1 GB big. Hard to do that when we have a very short time limit on the http request when operating in sync mode.
We have echo now. We should fix async upload by url. Possibly need to fix some bitrot too.
See T118887, r66438, r81612, r81612#c14654