jquery input type="file" change() only work once

Question

I want to use ajaxfileUpload to upload mutiple images to the server, so i user the callback change(),when input value change,i call the upload function, but it only work once successfully,do you have any ideas? here is the code:

<div class='popup_content clearfix' id="imagecontainer">
</div>
<script id="imagePreview" type="text/template">
  <div class='popup_img_container'>
<img class='popup_img' src="<%= imageUrl%>" id="imagepreview<%= imageCount %>">
  </div>
 </script>
<input type="file" id="postPic" name="fileToUpload"/>
var imageCount=0;

function uploadImage(){     

      alert( $("#postPic").val());
    $.ajaxFileUpload({
        url:'myurl',
        secureuri :false,
        fileElementId :"postPic",//file控件id
        dataType : 'json',
        success : function (data, status){
                  alert("done");
                  //$("#postAllpic").addClass("hidden");

        },
        error: function(data, status, e){
            alert(e);
        }
});
  }





$("#postPic").change(function(){
var objUrl = getObjectURL(this.files[0]) ;
if (objUrl) {
    var imageObj={};
    imageObj.imageUrl=objUrl;
    imageObj.imageCount=imageCount;
    var html=_.template($("#imagePreview").html(),imageObj);
    $("#imagecontainer").append(html);
    uploadImage();
    imageCount++;
    alert(imageCount);
}



});

 function getObjectURL(file) {
  var url = null ; 
 if (window.createObjectURL!=undefined) { // basic
    url = window.createObjectURL(file) ;
} else if (window.URL!=undefined) { // mozilla(firefox)
    url = window.URL.createObjectURL(file) ;
} else if (window.webkitURL!=undefined) { // webkit or chrome
    url = window.webkitURL.createObjectURL(file) ;
}
return url ;
}