How to Image Preivew using AjaxToolKit AsyncFileUpload

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Javascript

Ajax

ASP.NET4

Hi,
I want to upload image on server side folder using ajaxtoolkit ascyfileuploader but I want to preview that image and want to change name of that image before uploading it on server side.
here is the code..

<ajax:ToolkitScriptManager ID="scriptManager1"  runat="server"/>
    <div>
        <ajax:AsyncFileUpload ID="fileUpload1" OnClientUploadError="uploadError" 
        CompleteBackColor="White" Width="350px"  runat="server" UploaderStyle="Modern" 
        OnClientUploadComplete="uploadComplete" UploadingBackColor="#CCFFFF"  
        ThrobberID="imgLoad"  önUploadedComplete="fileUploadComplete" /><br />
        <asp:Image ID="imgLoad" runat="server" ImageUrl="loading.gif" />
        <asp:HiddenField ID="btnhidden" runat="server" />
        <br />

       
        <asp:Image ID="imgLoader" runat="server" ImageUrl="~/images/loader.gif" /><br /><br />
        <img id = "imgDisplay" alt="" src=""  style="display:none;"/>
    
    </div>
    <script type="text/javascript">
        function uploadComplete(sender, args) {

            var imgDisplay = $get("imgDisplay");
            imgDisplay.src = "images/loader.gif";
            imgDisplay.style.cssText = "";
            var img = new Image();
            img.onload = function () {
                imgDisplay.style.cssText = "height:100px;width:100px";
                imgDisplay.src = img.src;
            };

            <big></big>var myHidden = document.getElementById('<%= btnhidden.ClientID %>');
             var a = myHidden.value;

             img.src = "<%=ResolveUrl(UploadFolderPath) %>" + a;
        }
    </script>

protected void Page_Load(object sender, EventArgs e)
   {

   }
   protected string UploadFolderPath = "~/Files/";
   protected static int a = 0;
   protected void fileUploadComplete(object sender, AsyncFileUploadEventArgs e)
   {
           Thread.Sleep(1000);
           string filename = System.IO.Path.GetFileName(fileUpload1.FileName);
           string extension = Path.GetExtension(filename);
           string s = "abc" + a + "" + extension;

           btnhidden.Value = s;
           fileUpload1.SaveAs(Server.MapPath(this.UploadFolderPath) + s);
           a++;
   }

I am trying to set value to hidden filed on server side fileuploadcompleted event and access it on client side on completed event..
but on client side hidden filed value return null..
so please suggest me how can perform this whole operation.
thank you.