if(window.FileReader) { 
 var fr = new FileReader(); 
 // add your code here 
} 
else { 
 alert("Not supported by your browser!"); 
} 

<script type="text/javascript"> 
    function showPreview(source) { 
      var file = source.files[0]; 
      if(window.FileReader) { 
        var fr = new FileReader(); 
        fr.onloadend = function(e) { 
          document.getElementById("portrait").src = e.target.result; 
        }; 
        fr.readAsDataURL(file); 
      } 
    } 
  </script> 

 <input type="file" name="file" onchange="showPreview(this)" /> 
<img id="portrait" src="" width="70" height="75"> 

if(!/image\/\w+/.test(file.type)){ 
  alert("请确保文件为图像类型"); 
  return false; 
}