MVC：上传文件时限制文件类型

[HttpPost]
public ActionResult Upload(HttpPostedFileBase file)
{
    if (file == null)
    {
        return Content("没有文件！");
    }

    //-------------------------------------------------
    var thisExtension=Path.GetExtension(file.FileName);
    string[] allowExtension = new string[] {".xls",".xlsx" };
    if(!allowExtension.Contains(thisExtension))
    {
        return Content("文件类型不对。");
    }
    //-------------------------------------------------

    var filename = Path.Combine(Request.MapPath("~/upload"), Path.GetFileName(file.FileName));
    file.SaveAs(filename);
    return Content("OK");
}