public static class ExcelControllerExtensions
    {
        public static ActionResult Excel(this Controller controller, IEnumerable rows, Dictionary<string, string> headers, string[] footer, string fileName)
        {
            return new ExcelResult(rows, headers, footer, fileName);
        }
    }

    public class ExcelResult : ActionResult
    {
        public ExcelResult(IEnumerable rows, Dictionary<string, string> headers, string[] footer, string fileName)
        {
           //....
        }
        public override void ExecuteResult(ControllerContext context)
        {
            StringWriter sw = new StringWriter();
            HtmlTextWriter tw = new HtmlTextWriter(sw);
            //далее формируем HTML 
           //и записываем его в Response как экселевский файл

            HttpContext context = HttpContext.Current;
            context.Response.Clear();
            context.Response.AddHeader("content-disposition", "attachment;filename="+fileName);
            context.Response.Charset = "";
            context.Response.Cache.SetCacheability(HttpCacheability.NoCache);
            context.Response.ContentType = contentType;
            context.Response.Write(content);
            context.Response.End();

         }
}

[HttpGet]
public ActionResult ToExcel()
{
return this.Excel( items, headers, footer, "Report.xls");
}

contentType = "application/x-msdownload"

private void WriteFile(string fileName, string contentType, string content)
        {
            HttpResponse Response = HttpContext.Current.Response;

            Response.Clear();
            Response.ClearHeaders();
            Response.Expires = -1;
            Response.ContentType = contentType;
            Response.AddHeader("content-disposition", "attachment; filename=" + fileName);
            Response.AddHeader("Content-Length", content.Length.ToString());
            Response.BufferOutput = false;
            Response.ContentEncoding = System.Text.Encoding.UTF8;
            Response.Charset = "utf-8";
            Response.Flush();

            Response.BinaryWrite(System.Text.Encoding.UTF8.GetPreamble()); 
            Response.Write(content);
            Response.Flush();
            Response.End();
        }

            Response.ClearHeaders();