FileReference: Loading a Windows-locked file
A possible (dirty) workaround might be to wait for -let say- 10 seconds, and suppose that the file isn't available if no event has triggered then.
Using a setTimeout (and clearing it with clearTimeout in your COMPLETE and *_ERROR events handlers) might do the trick.
I'll be glad if someone could come up with a nicer solution, though.
EDIT: Of course you might want to listen to HTTP_STATUS event (waiting for a 202 answer - if I understood this documentation correctly) rather than waiting for COMPLETE or *_ERROR.