As I wrote in MBS-4376:

Once you've started uploading, there's no obvious way to stop it (I suppose closing the browser window would work but that's not ideal). I can imagine a couple of scenarios where you might want to:

• You suddenly notice after starting a large batch of images that you picked the wrong release. The sooner you can stop the upload, the fewer edits you'll have to cancel.
• You notice you made a mistake in something you haven't uploaded yet. Stopping the upload, fixing the mistake and then continuing would be the most natural way IMO for people to handle that. As it is, you would have to let it upload, cancel the edit and reupload the corrected image. Or submit an edit to fix the mistake and wait a week for it to be applied.