diff --git a/_config.php b/_config.php
index 6bfb33eb..f4db9dca 100644
--- a/_config.php
+++ b/_config.php
@@ -1,31 +1,28 @@
 <?php
 
 use SilverStripe\Admin\CMSMenu;
+use SilverStripe\CMS\Model\SiteTree;
 use SilverStripe\View\Parsers\ShortcodeParser;
-use SilverStripe\Assets\File;
-
 
 /**
  * - CMS_DIR: Path relative to webroot, e.g. "cms"
  * - CMS_PATH: Absolute filepath, e.g. "/var/www/my-webroot/cms"
  */
 define('CMS_PATH', realpath(__DIR__));
-if(strpos(CMS_PATH, BASE_PATH) === 0) {
-	define('CMS_DIR', trim(substr(CMS_PATH, strlen(BASE_PATH)), DIRECTORY_SEPARATOR));
+if (strpos(CMS_PATH, BASE_PATH) === 0) {
+    define('CMS_DIR', trim(substr(CMS_PATH, strlen(BASE_PATH)), DIRECTORY_SEPARATOR));
 } else {
-	throw new Exception("Path error: CMS_PATH " . CMS_PATH . " not within BASE_PATH " . BASE_PATH);
+    throw new Exception("Path error: CMS_PATH " . CMS_PATH . " not within BASE_PATH " . BASE_PATH);
 }
 
 /**
  * Register the default internal shortcodes.
  */
 ShortcodeParser::get('default')->register(
-	'sitetree_link',
-	array('SilverStripe\\CMS\\Model\\SiteTree', 'link_shortcode_handler')
+    'sitetree_link',
+    array(SiteTree::class, 'link_shortcode_handler')
 );
 
-File::add_extension('SilverStripe\\CMS\\Model\\SiteTreeFileExtension');
-
 // TODO Remove once we can configure CMSMenu through static, nested configuration files
 CMSMenu::remove_menu_class('SilverStripe\\CMS\\Controllers\\CMSMain');
 CMSMenu::remove_menu_class('SilverStripe\\CMS\\Controllers\\CMSPageEditController');
diff --git a/_config/config.yml b/_config/config.yml
index 5ee780e0..18c2551e 100644
--- a/_config/config.yml
+++ b/_config/config.yml
@@ -10,3 +10,4 @@ SilverStripe\Forms\Form:
 SilverStripe\Assets\File:
   extensions:
     - SilverStripe\CMS\Controllers\ErrorPageFileExtension
+    - SilverStripe\CMS\Model\SiteTreeFileExtension
diff --git a/code/Model/SiteTree.php b/code/Model/SiteTree.php
index 314ac110..5b483c33 100755
--- a/code/Model/SiteTree.php
+++ b/code/Model/SiteTree.php
@@ -995,7 +995,7 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
             return true;
         }
 
-        return $this->canEdit($member) && $this->stat('allowed_children') != 'none';
+        return $this->canEdit($member) && $this->stat('allowed_children') !== 'none';
     }
 
     /**
@@ -1160,7 +1160,7 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
      */
     public function canCreate($member = null, $context = array())
     {
-        if (!$member || !(is_a($member, 'SilverStripe\\Security\\Member')) || is_numeric($member)) {
+        if (!$member || !(is_a($member, Member::class)) || is_numeric($member)) {
             $member = Member::currentUserID();
         }
 
@@ -2713,25 +2713,36 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
      */
     public function allowedChildren()
     {
-        $allowedChildren = array();
-        $candidates = $this->stat('allowed_children');
-        if ($candidates && $candidates != "none" && $candidates != "SiteTree_root") {
-            foreach ($candidates as $candidate) {
-                // If a classname is prefixed by "*", such as "*Page", then only that class is allowed - no subclasses.
-                // Otherwise, the class and all its subclasses are allowed.
-                if (substr($candidate, 0, 1) == '*') {
-                    $allowedChildren[] = substr($candidate, 1);
-                } elseif ($subclasses = ClassInfo::subclassesFor($candidate)) {
-                    foreach ($subclasses as $subclass) {
-                        if ($subclass == 'SiteTree_root' || singleton($subclass) instanceof HiddenClass) {
-                            continue;
-                        }
-                        $allowedChildren[] = $subclass;
+        // Get config based on old FIRST_SET rules
+        $candidates = null;
+        $class = get_class($this);
+        while ($class) {
+            if (Config::inst()->exists($class, 'allowed_children', Config::UNINHERITED)) {
+                $candidates = Config::inst()->get($class, 'allowed_children', Config::UNINHERITED);
+                break;
+            }
+            $class = get_parent_class($class);
+        }
+        if (!$candidates || $candidates === 'none' || $candidates === 'SiteTree_root') {
+            return [];
+        }
+
+        // Parse candidate list
+        $allowedChildren = [];
+        foreach ($candidates as $candidate) {
+            // If a classname is prefixed by "*", such as "*Page", then only that class is allowed - no subclasses.
+            // Otherwise, the class and all its subclasses are allowed.
+            if (substr($candidate, 0, 1) == '*') {
+                $allowedChildren[] = substr($candidate, 1);
+            } elseif ($subclasses = ClassInfo::subclassesFor($candidate)) {
+                foreach ($subclasses as $subclass) {
+                    if ($subclass == 'SiteTree_root' || singleton($subclass) instanceof HiddenClass) {
+                        continue;
                     }
+                    $allowedChildren[] = $subclass;
                 }
             }
         }
-
         return $allowedChildren;
     }