Blank selected file field

[STEPHEN]

I have a button that uploads the selected file through RadInput.  However, if the user clicks it and the selected file input is blank it errors.  How can I check whether the RadInput is blank or null so I can send a custom error. Or can I hook it up to a validator? 

[STEPHEN]

found it:

1st a custom validator:

<asp:CustomValidator runat="server" id="CustomValidator1"
                                  display="Dynamic"
                                  ClientValidationFunction="validateRadUploadNoBlanks">
You have entered an invalid file. The file extension must be either XLS or XLSX! Please select another file!
                            </asp:CustomValidator>

Then JavaScript

<script type="text/javascript">
    function validateRadUploadNoBlanks(source, e) {
        e.IsValid = false;

        var upload = $find("<%= RadUpload1.ClientID %>");
        var inputs = upload.getFileInputs();
        for (var i = 0; i < inputs.length; i++) {
            //check for empty string or invalid extension
            if (inputs[i].value != "" && upload.isExtensionValid(inputs[i].value)) {
                e.IsValid = true;
                break;
            }
        }
    }
</script>

Then code behind on custom validator

Protected Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles CustomValidator1.ServerValidate
       'add this to the custom validator along with the java script function validateRadUploadNoBlanks
       args.IsValid = RadUpload1.UploadedFiles.Count > 0
   End Sub