- * Normalize a file extension to the common form, and ensure it's clean.
- * Extensions with non-alphanumeric characters will be discarded.
+ * Normalize a file extension to the common form, making it lowercase and checking some synonyms,
+ * and ensure it's clean. Extensions with non-alphanumeric characters will be discarded.
+ * Keep in sync with mw.Title.normalizeExtension() in JS.