code: public InputStream retrieveAttachment(SignalServiceAttachmentPointer pointer, File destination, int maxSizeBytes)
        throws IOException, InvalidMessageException
    {
      return retrieveAttachment(pointer, destination, maxSizeBytes, null);
    }
docs: Retrieves a SignalServiceAttachment.
  
  @param pointer The {@link SignalServiceAttachmentPointer}
  received in a {@link SignalServiceDataMessage}.
  @param destination The download destination for this attachment.
  
  @return An InputStream that streams the plaintext attachment contents.
  @throws IOException
  @throws InvalidMessageException