I have an Upload file option in Umbraco 7, the problem I have is if the remove checkbox is ticked and you click publish to remove the file, my code below catches the upload field as empty and display my custom error message.

if (string.IsNullOrEmpty(urlForFileUpload))
                {
                    e.CancelOperation(new EventMessage("Error", "Please select an Excel file before publishing",EventMessageType.Error));
                } 

Does anyone know what gets passed to the server so that I can check for it and not display my error message if the checkbox is ticked?