/etc/mediawiki-extensions/ is in mediawiki-extensions-base 3.6.
This file is owned by root:root, with mode 0o755.
..
/etc/mediawiki-extensions/extensions-available/
/etc/mediawiki-extensions/extensions-available/CategoryTree.php
/etc/mediawiki-extensions/extensions-available/Cite.php
/etc/mediawiki-extensions/extensions-available/CreateBox.php
/etc/mediawiki-extensions/extensions-available/Footnote.php
/etc/mediawiki-extensions/extensions-available/ImageMap.php
/etc/mediawiki-extensions/extensions-available/InputBox.php
/etc/mediawiki-extensions/extensions-available/LanguageSelector.php
/etc/mediawiki-extensions/extensions-available/NewUserNotif.php
/etc/mediawiki-extensions/extensions-available/NewestPages.php
/etc/mediawiki-extensions/extensions-available/News.php
/etc/mediawiki-extensions/extensions-available/PageCSS.php
/etc/mediawiki-extensions/extensions-available/Poem.php
/etc/mediawiki-extensions/extensions-available/Polyglot.php
/etc/mediawiki-extensions/extensions-available/RSSReader.php
/etc/mediawiki-extensions/extensions-available/SpecialCite.php
/etc/mediawiki-extensions/extensions-available/SpecialInterwiki.php
/etc/mediawiki-extensions/extensions-available/TreeAndMenu.php
/etc/mediawiki-extensions/extensions-enabled/
/etc/mediawiki-extensions/extensions.php