private @Nullable 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();
    }
  }
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.
jimfs/src/main/java/com/google/common/jimfs/FileSystemView.java