Merge pull request #13184 from owncloud/fix-key

Verify existence of $_GET key

apps/files/ajax/mimeicon.php

 <?php
 \OC::$server->getSession()->close();
 
-print OC_Helper::mimetypeIcon($_GET['mime']);
+$mime = isset($_GET['mime']) ? $_GET['mime'] : '';
+
+print OC_Helper::mimetypeIcon($mime);