Igor,
It's not a easy thing because starting upload is a client side behavior.
If you are not using AJAX , the client side state already be disposed.
So , if you use AJAX , you can just send JAVASCRIPT to start it :
var uploader=document.getElementById('<%=Uploader1.ClientID%>');
if(uploader.getqueuecount()>0)
{
uploader.startupload();
}
Regards,
Terry