Hi,
Currently the Uploader can't not be applied the validation control.
while the uploader would do a postback to server after it upload a file,
You can send a flag to client side after the file are uploaded.
Or you can check our online sample :
(it should be included in the download package)
This example show how to start upload manually,
Its code also show you how to check whether the user have selected files.
Regards,
Terry