Generalizes image upload in drag and drop in markdown to all files.

(w/ @erbunao)

This is our attempt to generalize the drag-and-drop upload in markdown areas. UI is still the same as #7011.  Note that only the images have valid preview.

@dosire, this PR doesn't have any blacklisted file types so we need suggestions from you. Thanks!
Can you please fix the indentation here?