code: private String getFileName(Field field) {
          String fileName = field.getAnnotation(Plugin.Data.class).value();
          if ("##default".equals(fileName)) {
              fileName = getName() + ".json";
          }
          return fileName;
      }
docs: Get file name for field with {@link Plugin.Data} annotation. If name
  is default then use {@link #name} as file name and <code>json</code>
  as file extension.
  
  @see #getName()