code: public void addFile(final FileWidgetUpload file) {
  		List<FileWidgetUpload> files = (List<FileWidgetUpload>) getData();
  		if (files == null) {
  			files = new ArrayList<>();
  			setData(files);
  		}
  		files.add(file);
  		MemoryUtil.checkSize(files.size(), this.getClass().getSimpleName());
  	}
docs: Add a file item to this widget.
  
  @param file the file item