The file namespace SHOULD be searched by default

Need to add to public/postLocalSettings.d/override.php (or into LocalSettings???)

$wgNamespacesToBeSearchedDefault = [
    NS_MAIN => true,
    NS_PROJECT => true,
    NS_FILE => true,
    NS_HELP => true,
    NS_CATEGORY => true

Checking the behaviour of the advanced search form on the demo site, it seems to find what you’re looking for (in the file namespace) on if FILE is checked /and/ you click another option (which seems to update the search parameters). See if this is a bug upstream, or a configuration problem.$wgNamespacesToBeSearchedDefault

The default is just NS_MAIN