Whenever post-back occurs, the posted-file property becomes blank, so people handle file control in many ways like
you can place a button (say Upload) near to file upload control, now user will select file and hit the button upload, on the click event of the upload button u can set the posted-file property to some hidden field, once user click on final submit button on the page, then u can upload the file to server.
Try
this[
^].
--Amit