File Upload Control doesn't keep Files

Question

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?