Added notice for File::getDefaultExtension() method

UPDATE.md

@@ -147,6 +147,9 @@ PR12 to beta1
 * The `File::getWebPath()` and `File::rename()` methods have been removed, as
   well as the `framework.document_root` configuration setting.
 
+* The `File::getDefaultExtension()` method has beean renamed to `File::guessExtension()`.
+  The renamed method now returns null if it cannot guess the extension.
+
 * The `session` configuration has been refactored:
 
   * The `class` option has been removed (use the `session.class` parameter