Author: Dongsheng Cai
MDL-26388, pop up a dialog when attaching a file already being used in draft area.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=f392caba154764f0afce93de3b544508b88be2ba
http://github.com/moodle/moodle/commit/f392caba154764f0afce93de3b544508b88be2ba