how?
http://stackoverflow.com/questions/9265788/how-to-get-the-server-path-to-the-web-directory-in-symfony2
http://stackoverflow.com/questions/9215753/how-to-get-the-root-dir-of-the-symfony2-application
http://stackoverflow.com/questions/12893657/how-to-get-web-directory-path-from-inside-entity
http://symfony.com/doc/current/cookbook/doctrine/file_uploads.html