Export to Excel Won't Work

I have a modal popup with a button control. When this button is clicked, I execute the following code which gets executed without any error but it won't ask to open or save the file. Also the excel file is not created, instead I see an diagnostic xml file having reference to this filename when I search for the file.

            Response.Clear();
            Response.Buffer = true;
            Response.ContentType = "application / vnd.ms - excel";
            Response.AddHeader("content - disposition", "attachment; filename = MyFiles.xls");
            Response.Charset = "";
            this.EnableViewState = false;

            System.IO.StringWriter sw = new System.IO.StringWriter();
            System.Web.UI.HtmlTextWriter htw = new System.Web.UI.HtmlTextWriter(sw);

            dgCurrentPoBdetails.RenderControl(htw);

            Response.Write(sw.ToString());
            Response.End();

I've tried using the following code; it does create an Excel file but does not prompt me to open or save. Also, I am not able to open up this file; it give me an error "Excel cannot open this file because the file format of the file extension is invalid"

            System.IO.StringWriter sw = new System.IO.StringWriter();
            System.Web.UI.HtmlTextWriter htw = new System.Web.UI.HtmlTextWriter(sw);

            // Render grid view control.
            dgCurrentPoBdetails.RenderControl(htw);

            // Write the rendered content to a file.
            string renderedGridView = sw.ToString();
            System.IO.File.WriteAllText(@"C:\MoQ\ExportedFile.xlsx", renderedGridView);
            string myStringVariable = "File saved to: " + "C:\\MyPath\\ExportedFile.xlsx";
            ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "alertMessage", "alert(" + myStringVariable + ")", true);