@Override
  public @Nullable Object get(File file, String attribute) {
    switch (attribute) {
      case "group":
        return file.getAttribute("posix", "group");
      case "permissions":
        return file.getAttribute("posix", "permissions");
      default:
        return null;
    }
  }
Attribute provider that provides the {@link PosixFileAttributeView} ("posix") and allows reading
of {@link PosixFileAttributes}.

@author Colin Decker
jimfs/src/main/java/com/google/common/jimfs/PosixAttributeProvider.java