.APB extension file have to read as a string (data) in .NET (2.0) C#

txt_box.Text = "";
        if (FileUpload1.HasFile)
        {

            string ServerMapPath = Server.MapPath("~//images//" + FileUpload1.FileName);

            FileUpload1.PostedFile.SaveAs(ServerMapPath);
            //string x1=FileUpload1.PostedFile.InputStream.ToString();
            //Response.Write(x1);

            StreamReader sr = new StreamReader(ServerMapPath);

            string file_data;
            string sb = "";
            while ((file_data = sr.ReadLine()) != null)
            {
                //Response.Write(file_data);
                sb = sb + file_data;

            }
            txt_box.Text = sb;
            
            sr.Close();
            File.Delete(ServerMapPath);

        }
        else
        {
            txt_box.Text ="Upload a file then click";
        }