It is possible but not simple.
(I have not done it.)
The article referred to by Jonathon covers the basic of file uploading in
..Net.
Using those techniques you *cannot* do what you want.
You need to write more advanced code and intercept the http request and
handle it yourself.
If you have complete control then you can do what you want.
There was huge discussion of this issue a couple of years ago and many
developers worked out 90% of the code to handle it and then declared the
rest "trivial". (Maybe for them but not me!)
anyway - some of them sold their work to companies that publish controls to
handle this. They are quite expensive.
I wish someone had completed the code and posted a 100% working sample.
I think this is the (very long) discussion:
http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=55127