trigger change event for file field while uploading pdf

STUD-1775
@waheedahmed @symbolist @adampalay 
Make File upload modal to trigger change event when a new file is selected. Currently modal thinks that no change is made (just another File object so ignore it) when user selects a new file after selecting first time.
Please add a comment that this is necessary for FireFox (the bug does not reproduce for me with Chrome on Mac-- only Firefox. For Chrome, this.model.changed is correctly populated).