def _write_mef(self, key, extlist, outfile):
        """Write out regular multi-extension FITS data."""
        channel = self.fv.get_channel(self.chname)
        with fits.open(outfile, mode='update') as pf:
            # Process each modified data extension
            for idx in extlist:
                k = '{0}[{1}]'.format(key, self._format_extname(idx))
                image = channel.datasrc[k]

                # Insert data and header into output HDU
                pf[idx].data = image.get_data()
                self._write_header(image, pf[idx])

            # Write history to PRIMARY
            self._write_history(key, pf['PRIMARY'])
Write out regular multi-extension FITS data.