Selected file missing after page load

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Hi all,
I got a problem to restore back the selected upload
file after i fired a radio button that will call the
server script to do the process.
In my aspx page, i have an upload textbox to allow users
to upload the file. Under the textbox, there have 2 radio
button. The process is i select the file first, and then
i choose one of the radio button, after the radio button
is fired, it will have a server script function to do
some process. After the process is finished, the selected
file dissapeared from the textbox. I need to reselect the
file again in order to upload it to the server.
Does anybody knows how do eliminate the file missing
after i choose the radio button?


Thanks a lot.