nextcloud/apps/files_versions/appinfo/routes.php

<?php
/**
 * Copyright (c) 2013, Tom Needham <tom@owncloud.com>
 * This file is licensed under the Affero General Public License version 3 or later.
 * See the COPYING-README file.
 */

/** @var $this \OCP\Route\IRouter */
$this->create('core_ajax_versions_preview', '/preview')->action(
function() {
	require_once __DIR__ . '/../ajax/preview.php';
});

// Register with the capabilities API
OC_API::register('get', '/cloud/capabilities', array('OCA\Files_Versions\Capabilities', 'getCapabilities'), 'files_versions', OC_API::USER_AUTH);