Filter Article View by Extension.

The administrator category list view filters by extension, which allows 3rd party extensions to use the #__categories database table.

However the article list view does not filter by extension type. This means that 3rd party extensions using the #__content database table for their content items appear in Joomla's article list. The Front-End code does properly filter by extension which enforces why this is expected behaviour.

The fix I propose copies the exact filtering code used in the com_categories\models\categories.php model. Although I did notice that the categories model didn't filter the #__associations query in getListQuery(); by extension so I have also left this out when copying the logic across to the articles model.