I think the best way, if you can change your code a bit, is to add plugins to add files, for example:
http://swfupload.org/
or
http://www.uploadify.com
they are more common, I will work with the first, and I had no problems.
This will not do what you want, but you can add files to the queue and then cancel them, and you can capture this event ...
I know that this is not what you were looking for, but it is the best solution I can give you.
Good luck I hope I helped you.
MCSI
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