through the code in the viewonline.php is

preg_match('#^([a-z/]+)#i', $row['session_page'], $on_page);

the switch case won't work for files including a _. This is not the case on phpbb itself, but on some mods. This could be easily fixed by adding the _ to the preg_match

preg_match('#^([a-z/_]+)#i', $row['session_page'], $on_page);