This add-on is a wrapper around the Vaadin Upload component that allows to add a server validator to decide if the uploaded file is valid to be accepted from the content point of view. If the validation fails, the respective file is marked with the validation error message on the client. Using the server validation: ``` upload.addFileValidator((filename, mimeType) -> { //do some server validation with the uploaded file and decide if the file is valid or not ... server validation check ... return ValidationResult.error("File is of wrong template/format!"); }); ```