..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``