@Test
  public void extension() {
    assertThat(Resources.extension(Paths.get("file.txt"))).isEqualTo(".txt");
    assertThat(Resources.extension(Paths.get("file.css.map"))).isEqualTo(".map");
    assertThat(Resources.extension(Paths.get(".dotfile.ext"))).isEqualTo(".ext");

    assertThat(Resources.extension(Paths.get("file"))).isEmpty();
    assertThat(Resources.extension(Paths.get(".dotfile"))).isEmpty();
    assertThat(Resources.extension(Paths.get("."))).isEmpty();
  }