Behavior of multiple option differs in UI mode and Core mode

[rnicholus]

When the multiple option is false, a file is in-progress, and a new file is selected...

In UI mode: The in progress upload is cancelled, and replaced in the UI w/ the new file.
In Core mode: The in progress upload is not cancelled.

The documentation points out this difference, but it's not clear why this difference is desirable. I believe we should be more consistent, and the in-progress upload should be cancelled in all cases if a new file is selected when the multiple option is false.

Discovered while writing up #1039.