If System.IO.File.Exists and OnClientClick event

Hi,
I have a button that checks if a file already exists or not in a folder.
I want a OnClientClick event to ONLY fire if there is a file(the same file) in the folder and ask me if i want to overwrite the file or not ( if press OK then overwrite if Cancel the do nothing) 
 
something like this.             If System.IO.File.Exists(UploadFile) Then                 onclientclick="return confirm('Are you sure you want to overwrite this file?')"
            Else                 FileUploadBrowser.SaveAs(UploadF ...