<script>  
 function focusAsyncButton(sender, args) {
                    var inputs = $telerik.$(".ruFileInput");

                    var input = inputs[inputs.length - 1];

                    if (input) {
                        input.focus();
                    }
                }
</script>

<telerik:RadAjaxManager ID="RadAjaxManager1" runat="server" ClientEvents-OnResponseEnd="focusAsyncButton">