File Upload Control doesn't keep Files

Hello Experts,
i have a problem using the fileupload control,
Scenario :
i have a preview button which allow the user to preview the uploaded file,the preview button opens a new window with the content of that file,but when i close the popup window the fileuploader no longer have the file,so i have to choose it again...

here is the code for the preview button:

protected void PreviewLink_Click(object sender, EventArgs e)

{

if (ReportUpload.HasFile == true)

{

if (Path.GetExtension(ReportUpload.PostedFile.FileName) == ".csv")

{

String FileSavePath = Server.MapPath("~/Uploads/Reports/Files/");

String FileName = ReportUpload.FileName;

String FullFilePath = FileSavePath + FileName;

ReportUpload.SaveAs(FullFilePath);

Session["CSVPreview"] = Excel.ConvertToHtmlFile(Excel.ReadExcel(FileSavePath, ".csv"));

string script = "window.open('"+"ExcelPreview.aspx"+"','')";

if (!ClientScript.IsClientScriptBlockRegistered("NewWindow"))

{

ClientScript.RegisterClientScriptBlock(this.GetType(), "NewWindow", script, true);

}

any ideas?