diff -u b/core/lib/Drupal/Core/Config/FileStorage.php b/core/lib/Drupal/Core/Config/FileStorage.php
--- b/core/lib/Drupal/Core/Config/FileStorage.php
+++ b/core/lib/Drupal/Core/Config/FileStorage.php
@@ -63,7 +63,7 @@
    *   The path to the configuration file.
    */
   public function getFilePath() {
-    return $this->getPath() . '/' . $this->getName() . '.' . $this->getFileExtension();
+    return $this->getPath() . '/' . $this->getName() . '.' . self::getFileExtension();
   }
 
   /**
only in patch2:
unchanged:
--- a/core/modules/system/config/system.rss-publishing.xml
+++ /dev/null
@@ -1,6 +0,0 @@
-<?xml version="1.0"?>
-<config>
-  <feed_description></feed_description>
-  <feed_default_items>10</feed_default_items>
-  <feed_item_length>fulltext</feed_item_length>
-</config>
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/config/system.rss-publishing.yml
@@ -0,0 +1,3 @@
+feed_description:
+feed_default_items: '10'
+feed_item_length: 'fulltext'