Asp.net c# ทําไมเวลากด Button Event เพื่อ Post ค่าไปทำอะไรสักอย่างแล้ว มันวนกลับมาหน้าฟอร์มเดิมแต่กด Refresh F5 error

public partial class Default : System.Web.UI.Page
    {
        StreamReader sr;
        string sql;
        protected void Page_Load(object sender, EventArgs e)
        {
            //if(!Page.IsPostBack)
            //{
            //    LoadData();
            //}

            LoadData();
        }

        private void LoadData()
        {
            sql = "SELECT * FROM FingerScanData";
            Database.cmd = new SqlCommand(sql, Database.conn);
            Database.adap = new SqlDataAdapter(Database.cmd);
            DataTable dt = new DataTable();
            Database.conn.Open();
            Database.adap.Fill(dt);
            Database.conn.Close();

            GvFileResult.DataSource = dt;
            GvFileResult.DataBind();
        }

        protected void GvFileResult_OnRowCommand(object sender, GridViewCommandEventArgs e)
        {
            string file = e.CommandArgument.ToString();
            Response.ClearContent();
            Response.Clear();
            Response.ContentType = "text/plain";
            Response.AddHeader("Content-Disposition", "attachment; filename=" + file + ";");
            Response.TransmitFile(Server.MapPath("FilesConvert/") + file);
            Response.Flush();
            Response.End();

            Response.Write("<script>");
            Response.Write("alert('Success');");
            Response.Write("</script>");
        }

        protected void GvResult_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            GvFileResult.PageIndex = e.NewPageIndex;
            GvFileResult.DataBind();
        }

        protected void btnUpload_Click(object sender, EventArgs e)
        {
            if (Page.IsPostBack)
            {
                if (this.FileUpload.HasFile)
                {
                    if (Path.GetExtension(FileUpload.FileName) == ".txt")
                    {
                        string folderPath = Server.MapPath("Files/");
                        string folderPathConvert = Server.MapPath("FilesConvert/");
                        string oldFilename = FileUpload.FileName;
                        string newFilename = "TUS" + DateTime.Now.ToString("yyyyMMddhhmmss") + ".txt";

                        try
                        {
                            this.FileUpload.SaveAs(folderPath + FileUpload.FileName);
                            File.Move(folderPath + FileUpload.FileName, folderPath + newFilename);
                            using (StreamWriter sw = (!File.Exists(folderPathConvert + newFilename)) ? File.CreateText(folderPathConvert + newFilename) : new StreamWriter(folderPathConvert + newFilename))
                            {
                                sr = File.OpenText(folderPath + newFilename);
                                while (!sr.EndOfStream)
                                {
                                    string[] dataSplit = sr.ReadLine().Split('\t');
                                    string[] date = dataSplit[4].Split('-');
                                    string[] time = dataSplit[5].Split(':');
                                    int hour = Int32.Parse(time[0]);
                                    sw.WriteLine("TUS" + dataSplit[0] + " " + date[0] + date[1] + date[2] + hour.ToString("00") + time[1] + time[2]);
                                }
                                sw.Close();
                            }

                            sql = "INSERT INTO FingerScanData (Company, OldFilename, Filename) VALUES (@Company, @OldFilename, @Filename);";
                            Database.cmd = new SqlCommand(sql, Database.conn);
                            Database.cmd.Parameters.AddWithValue("@Company", "TUS");
                            Database.cmd.Parameters.AddWithValue("@OldFilename", oldFilename);
                            Database.cmd.Parameters.AddWithValue("@Filename", newFilename);
                            Database.conn.Open();
                            Database.cmd.ExecuteNonQuery();
                            Database.conn.Close();

                            this.lblText.Text = "Upload Success";
                            LoadData();
                        }
                        catch (Exception ex)
                        {
                            this.lblText.Text = "Read Failed" + ex.Message;
                        }
                    }
                    else
                    {
                        this.lblText.Text = "File type incorrect!";
                    }
                }
                else
                {
                    this.lblText.Text = "Select File";
                }
            }
        }
    }