@GuardedBy("this")
  @CheckForNull
  private File file;

  /** ByteArrayOutputStream that exposes its internals. */
  private static class MemoryOutput extends ByteArrayOutputStream {
    byte[] getBuffer() {
      return buf;
    }

    int getCount() {
      return count;
    }
  }

  /** Returns the file holding the data (possibly null). */
  @VisibleForTesting
  @CheckForNull