nextcloud-server/apps/files/ajax/download.php

<?php
/**
 * @author Andreas Fischer <bantu@owncloud.com>
 * @author Bart Visscher <bartv@thisnet.nl>
 * @author Björn Schießle <schiessle@owncloud.com>
 * @author Frank Karlitschek <frank@owncloud.org>
 * @author Jörn Friedrich Dreyer <jfd@butonic.de>
 * @author Lukas Reschke <lukas@owncloud.com>
 * @author Morris Jobke <hey@morrisjobke.de>
 * @author Robin Appelman <icewind@owncloud.com>
 *
 * @copyright Copyright (c) 2016, ownCloud, Inc.
 * @license AGPL-3.0
 *
 * This code is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License, version 3,
 * as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License, version 3,
 * along with this program.  If not, see <http://www.gnu.org/licenses/>
 *
 */

// Check if we are a user
OCP\User::checkLoggedIn();
\OC::$server->getSession()->close();

$files = isset($_GET['files']) ? (string)$_GET['files'] : '';
$dir = isset($_GET['dir']) ? (string)$_GET['dir'] : '';

$files_list = json_decode($files);
// in case we get only a single file
if (!is_array($files_list)) {
	$files_list = array($files);
}

/**
 * this sets a cookie to be able to recognize the start of the download
 * the content must not be longer than 32 characters and must only contain
 * alphanumeric characters
 */
if(isset($_GET['downloadStartSecret'])
	&& !isset($_GET['downloadStartSecret'][32])
	&& preg_match('!^[a-zA-Z0-9]+$!', $_GET['downloadStartSecret']) === 1) {
	setcookie('ocDownloadStarted', $_GET['downloadStartSecret'], time() + 20, '/');
}

OC_Files::get($dir, $files_list, $_SERVER['REQUEST_METHOD'] == 'HEAD');