js 检测上传图片类型与高宽（按比例缩放）

<form action="" method="post" enctype="multipart/form-data" name="dform">
  <label for="ifile"></label>
  <input type="file" name="ifile" id="ifile">
  <input type="submit" name="button" id="button" value="提交" onclick="网页特效:toi_upimg();">
</form>
<script>
function toi_upimg()
{
  var form = document.dform;
  var isrc = form.ifile.value;
  var pos = isrc.lastindexof('.');
  var img = new image();

  // on load
  img.onload = function () {
    var width = img.width;
    var height = img.height;
    img.onload = '';
    if (width > 480 || height > 320)
    {
      // => 480*360
      var ratio1 = 320/width;
      var ratio2 = 240/height;
      if (ratio1 > ratio2)
      {
        width = parseint(ratio2 * width);
        height = 240;
      }
      else
      {
        width = 320;
        height = parseint(ratio1 * height);
      }     
      alert('注意：图片过大, 系统自动进行了一些调整');
 }
   
    toi_img.src = img.src;
    toi_img.width = width;
    toi_img.height = height;
  form.picwidth.value = width;
  form.picheight.value = height;
  form.mserver.value = "no";
    form.xpost.value = '0';
    form.ypost.value = '0';
    form.xposy.value = '0';
    form.yposy.value = '0';
    form.xposa.value = '0';
    form.yposa.value = '0';
 if (form.title.value == '')
    {
      var pos2 = isrc.lastindexof('');
      form.title.value = isrc.substring(pos2+1,pos);
    }
    toi_update();
    delete img;
  };

  // on error
    img.onerror = function () {
    img.onerror = '';
    alert('注意：您刚刚试图上载的一个文件似乎不是合法图片');
    delete img;
  };

  // store the source address
  if (navigator.appversion.indexof('msie 7') >= 0)
  {
   document.getelementbyid("filterimage").filters.item("dximagetransform.microsoft.alphaimageloader").src = isrc;
   img.src = document.getelementbyid("filterimage").filters.item("dximagetransform.microsoft.alphaimageloader").src;
  }
  else
  {
   img.src = isrc;
  }
}
</script>