Cancelling a (big) file upload doesn't work as expected

In a folder, click "Import a file".
Select a file and click "upload" -> the progress bar starts filling up.
Hit "cancel" -> the progress bar resets to "almost empty", nothing else happens. Clicking "stop" doesn't change anything.

Possible cause:
The file upload isn't actually interrupted, those actions will only take effect when the file upload is complete.
If this is the case, this will be much more noticeable with big files from a remote location.