Get Handle to File Download Box

Hi,

In my application, on the click of a button, a popup-window shows up
which tries to download a file from the server. When the file is
downloaded, the 'Open/Save' dialog appears and the user makes his
choice. This works fine. My problem is that once the user makes his
choice (open or save the file), I want to close the pop-up window, but
I don't know how to detect that user has made his choice so that I can
fire window.close() on the window. Can anybody help?

Thanks.
Puneet