server/core/ajax/preview.php

<?php
/**
 * Copyright (c) 2013 Georg Ehrke georg@ownCloud.com
 * This file is licensed under the Affero General Public License version 3 or
 * later.
 * See the COPYING-README file.
 */
\OC_Util::checkLoggedIn();
\OC::$server->getSession()->close();

$file = array_key_exists('file', $_GET) ? (string)$_GET['file'] : '';
$maxX = array_key_exists('x', $_GET) ? (int)$_GET['x'] : '36';
$maxY = array_key_exists('y', $_GET) ? (int)$_GET['y'] : '36';
$scalingUp = array_key_exists('scalingup', $_GET) ? (bool)$_GET['scalingup'] : true;
$keepAspect = array_key_exists('a', $_GET) ? true : false;
$always = array_key_exists('forceIcon', $_GET) ? (bool)$_GET['forceIcon'] : true;

if ($file === '') {
	//400 Bad Request
	\OC_Response::setStatus(400);
	\OC_Log::write('core-preview', 'No file parameter was passed', \OC_Log::DEBUG);
	exit;
}

if ($maxX === 0 || $maxY === 0) {
	//400 Bad Request
	\OC_Response::setStatus(400);
	\OC_Log::write('core-preview', 'x and/or y set to 0', \OC_Log::DEBUG);
	exit;
}

$info = \OC\Files\Filesystem::getFileInfo($file);

if (!$info instanceof OCP\Files\FileInfo || !$always && !\OC::$server->getPreviewManager()->isAvailable($info)) {
	\OC_Response::setStatus(404);
} else {
	$preview = new \OC\Preview(\OC_User::getUser(), 'files');
	$preview->setFile($file);
	$preview->setMaxX($maxX);
	$preview->setMaxY($maxY);
	$preview->setScalingUp($scalingUp);
	$preview->setKeepAspect($keepAspect);
	$preview->showPreview();
}