Added data-reset to help server side code keep track of removed images

Simple change to set a checkbox to 'on' if the fileupload form has been cleared, and 'off' otherwise. Needs a checkbox input with 'data-reset' attribute to work.

The main reason for doing this is so it would be easier to integrate with frameworks like django where the image preview can be set to a previously uploaded image and then pressing Remove will actually delete it from the server, rather than having a special 'delete' or 'clear' button not related to the Fileupload UI.