Adding a note thanks to Stof

Author: weaverryan

cookbook/form/create_form_type_extension.rst

@@ -127,7 +127,9 @@ tag:
 
 .. versionadded:: 2.8
     The ``extended_type`` option is new in Symfony 2.8. Before, the option was
-    called ``alias``.
+    called ``alias``. If you're a bundle author and need to support multiple Symfony
+    versions, specify *both* ``extended_type`` and ``alias`` (having both will prevent
+    any deprecation warnings).
 
 The ``extended_type`` key of the tag is the type of field that this extension should
 be applied to. In your case, as you want to extend the ``Symfony\Component\Form\Extension\Core\Type\FileType``