hes input to file <strong>fn</strong>.
        Returns 0 on success and error code on error.<br>
        The function used to read metadata from external JPEG file (on cameras
        with "Diag RAW hack"). <br>
        This call is not implemented for <a href="#buffer_datastream">LibRaw_buffer_datastream</a>,
        so external JPEG processing is not possible when buffer datastream used.
        <br>
        This function should be implemented in real input class, base class call
        always return err