It's not work for any value. We have problem with all MU of plesk that overwrite our change in config horde file (we always active html view, default horde (and plesk update) always overwrite the change to OFF), the correct way seem to create an /usr/share/psa-horde/imp/config/mime_drivers.local.php and /usr/share/psa-horde/config/mime_drivers.local.php that are copy of /usr/share/psa-horde/imp/config/mime_drivers.php and /usr/share/psa-horde/config/mime_drivers.php, with changed parameters.