@VisibleForTesting
  @CheckForNull
  synchronized File getFile() {
    return file;
  }

  /**
   * Creates a new instance that uses the given file threshold, and does not reset the data when the
   * {@link ByteSource} returned by {@link #asByteSource} is finalized.
   *
   * @param fileThreshold the number of bytes before the stream should switch to buffering to a file