code: @SuppressWarnings("unchecked")
    public <T> T fileField(String key) {
      T result = null;
      Map<String, Object> file = getField("file");
      if (file != null) {
        result = (T) file.get(key);
      }
      return result;
    }
docs: Helper method to extract a field from the {@code file} map.
  
  @param key the key who's value to be returned.
  @param <T> the type of this field.
  @return field of this file.