edit-gallery.php

<?php

function attachments_meta_box() {

		$work_sample = new KOAO_Work_Sample( get_post() );
		$attachments = wp_list_pluck( $work_sample->attachments(), 'ID' );

		wp_enqueue_media();

		$js = <<<JS
jQuery(document).ready(function ($) {
	$('[data-action="edit-gallery"]').click(function (event) {
		event.preventDefault();
		wp.media.gallery.edit('[gallery ids="'+ $(this).next().val() +'"]')
			.on('update', function(collection) {
	    		$(this).next().val( collection.pluck('id').concat(',') );
	    	}.bind(this));
	});
});
JS;
		wp_add_inline_script( 'media-editor', $js );
		?>

		<p class="hide-if-no-js">
			<a href="#" data-action="edit-gallery"><?php esc_html_e( 'View / Edit Attachments', 'koao-theme' ); ?></a>
			<input type="hidden" name="koao_work_sample[gallery_ids]"
			       value="<?php echo esc_attr( join( ',', $attachments ) ); ?>"/>
		</p>
		<?php

	}