Currently, NS_FILE does not have subpages enabled.
Defines.php includes define( 'NS_FILE', 6 );
A number of wikis listed in wgNamespacesWithSubpages specifically declare that there should not be subpages in ns 6 (6 => 0) despite this already being the case. There are other such duplications, but this is the most common. These statements are extraneous and should be removed, both for code readability and because they unnecessarily bloat the size of InitialiseSettings.php.