why file path of uploaded file is undefined ?

Question

I have ajax Async File Upload Control in asp.net, inside update panel. I have called javascript function on it to get filename of uploaded file but it always give Undefined text. Why ?

<asp:AsyncFileUpload ID="FileUpload1" OnClientUploadComplete="uploadComplete" ClientIDMode="AutoID" UploaderStyle="Modern" runat="server"/>
asp:HiddenField ID="HdnFieldEmployeePicture" runat="server" />
<asp:HiddenField ClientIDMode="Static" ID="HdnFieldHasFileUploaded" runat="server" />
<asp:HiddenField ClientIDMode="Static" ID="UploadedFilePath" runat="server" />

calling this function:

<script type="text/javascript">
        function uploadComplete(sender, args) {
            var myHidden = document.getElementById('<%= HdnFieldHasFileUploaded.ClientID %>');
            myHidden.value = '1';

            var fu1 = args.get_fileName();
            UploadedFilePath.value = fu1.value;

        }
    </script>

fu1 is always UNDEFINED, why ?