code: @Nullable
    private RegularFile lookUpRegularFile(JimfsPath path, Set<OpenOption> options)
        throws IOException {
      store.readLock().lock();
      try {
        DirectoryEntry entry = lookUp(path, options);
        if (entry.exists()) {
          File file = entry.file();
          if (!file.isRegularFile()) {
            throw new FileSystemException(path.toString(), null, "not a regular file");
          }
          return open((RegularFile) file, options);
        } else {
          return null;
        }
      } finally {
        store.readLock().unlock();
      }
    }
docs: Looks up the regular file at the given path, throwing an exception if the file isn't a regular
  file. Returns null if the file did not exist.