Excel文件的格式与文件扩展名指定的格式不同(Excel从gridview导出)

我正在将网格视图导出到excel文件中，它打开得很好。唯一的问题是，每次打开excel文件时都会弹出此警告：

The file you are trying to open < > is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?

我正在使用的代码：

        HttpContext.Current.Response.Clear();
        HttpContext.Current.Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", "Single_Raw.xls"));
        HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";
    using (StringWriter sw = new StringWriter())
    {
        using (HtmlTextWriter htw = new HtmlTextWriter(sw))
        {
           // some code
            HttpContext.Current.Response.Write(sw.ToString());
            HttpContext.Current.Response.End();
        }
    }