主要用于上传图片的过程中可以显示图片，这样就可以确认图片有没有远错，利用提高正确率。

<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>

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