<script language="javascript" type="text/javascript">
 var allowExt = ['jpg', 'gif', 'bmp', 'png', 'jpeg'];
 var preivew = function(file, container){
  try{
  var pic = new Picture(file, container);
  }catch(e){
  alert(e);
  }
 }
 //缩略图类定义
   var Picture = function(file, container){
  var height = 0, 
  widht = 0, 
  ext = '',
  size = 0,
  name  = '',
  path  = '';
  var self = this;
  if(file){ 
    name = file.value;
  if (window.navigator.userAgent.indexOf("MSIE")>=1){ 
   file.select(); 
   path = document.selection.createRange().text; 
  }else if(window.navigator.userAgent.indexOf("Firefox")>=1){ 
   if(file.files){ 
   path = file.files.item(0).getAsDataURL(); 
   }else{
   path = file.value;
   }
  } 
  }else{
  throw "bad file";
  } 


  ext = name.substr(name.lastIndexOf("."), name.length);

  if(container.tagName.toLowerCase() != 'img'){
  throw "container is not a valid img label";
  container.visibility = 'hidden';
  }
  container.src = path;
  container.alt = name;
  container.style.visibility = 'visible';
  height = container.height;
  widht = container.widht;
  size  = container.fileSize;


  this.get = function(name){
  return self[name];
  }

  this.isValid = function(){
  if(allowExt.indexOf(self.ext) !== -1){
   throw 'the ext is not allowed to upload';
   return false;
  }
  }
 }
</script>
  <div class='previewDemo'>
 <input id="file" type="file" onchange="preivew(this, document.getElementById('img'));">
 <img id="img" style="visibility:hidden" height="100px" width="100px">
</div>