Catch user navigating away while upload in progress

Currently the file upload will either be aborted or the file status will be left false when the user navigates away form the page. If the file status is false, the file fill be automatically deleted after cron. To add an oBeforeUnload callback won't be solid enough AFAIK, while you can't catch this event if the browsers window is closed. The 'division by zero' warning seems to be a bug. I will look at that asap.