Telerik RadAsyncUpload after validation file missing

Question

May I ask about How to keep the file after validation for Telerik RadAsync Upload.

<telerik:RadScriptManager runat="server" ID="RadScriptManager"></telerik:RadScriptManager>         
<telerik:RadAsyncUpload RenderMode="Lightweight" runat="server" ID="RadAsyncUpload1"></telerik:RadAsyncUpload>

I had put the Ajax settings, but its not work.

<telerik:RadAjaxManager runat="server" ID="RadAjaxManager1">

            <AjaxSettings>
                <telerik:AjaxSetting AjaxControlID="RadAsyncUpload1">
                    <UpdatedControls>
                        <telerik:AjaxUpdatedControl ControlID="RadAsyncUpload1" />
                    </UpdatedControls>
                </telerik:AjaxSetting>

            </AjaxSettings>
        </telerik:RadAjaxManager>

Answer 1

Here is the client validation function of the validator:

<telerik:RadAsyncUpload ID="RadAsyncUpload2" runat="server" Skin="Outlook" MultipleFileSelection="Automatic"></telerik:RadAsyncUpload>

<asp:CustomValidator runat="server" ID="CustomValidator1" ClientValidationFunction="validateUpload2"
    ErrorMessage="This is Required Field" ForeColor="Red"></asp:CustomValidator>
<script type="text/javascript">

    function validateUpload2(sender, args) {
        var upload = $find("<%=RadAsyncUpload2.ClientID%>");
        args.IsValid = upload.getUploadedFiles().length != 0;
    }
</script>

Here is the server validation function of the validator:

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
{
   args.IsValid = (RadAsyncUpload2.UploadedFiles.Count == 0);
}