限制上传文件为jpg文件

function checkPhoto(fnUpload)
{
  var filename = fnUpload.value;
  alert(filename);
  var mime = filename.toLowerCase().substr(filename.lastIndexOf("."));
  if(mime!=".jpg")
  {
      alert("请选择jpg格式的照片上传");
      fnUpload.outerHTML=fnUpload.outerHTML;
  }
}