code: @Nullable
      protected final String generateEntityTag(HttpFileAttributes attrs) {
          requireNonNull(attrs, "attrs");
          return entityTagFunction != null ? entityTagFunction.apply(pathOrUri(), attrs) : null;
      }
docs: Generates an entity tag of the file with the given attributes using the {@code entityTagFunction}
  which was specified with the constructor.
  
  @return the entity tag or {@code null} if {@code entityTagFunction} is {@code null}.