图片上传即时显示缩略图解决方法（出自高手“风雪之隅” ）

<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>