<form enctype="multipart/form-data" id="oForm">
    <input type="file" name="file" id="file" onchange="readAsDataURL()" />
    <input type="button" value="提交" onclick="doUpload()" />
  </form>
  <div>
    <img alt="" id="img"/>
  </div>

  function doUpload() { 
     var formData = new FormData($( "#oForm" )[0]); 
     console.log(formData); 
     $.ajax({ 
       url: 'pp', 
       type: 'POST', 
       data: formData, 
       async: false, 
       cache: false, 
       contentType: false, 
       processData: false, 
       success: function (returndata) { 
         console.log(returndata); 
       }, 
       error: function (returndata) { 
         console.log(returndata); 
       } 
     }); 
  } 

function readAsDataURL(){
  //检验是否为图像文件
    var file = document.getElementById("file").files[0];
    if(!/image\/\w+/.test(file.type)){
      alert("看清楚，这个需要图片！");

      return false;
    }else{
    var reader = new FileReader();
    //将文件以Data URL形式读入页面
    reader.readAsDataURL(file);
    reader.onload=function(e){
      var result=document.getElementById("img");
      //显示文件
      result.src= this.result ;
    }
  }
}