How to add additional non-entity fields to entity form in Symfony2

Use the property_path option:

$builder->add('image', 'file', [
    'property_path' => false,
]);

Use mapped:

$form = $this->createFormBuilder($promo)
    ->add('code', 'text')
    ->add('image', 'file', array(
                "mapped" => false,
            ))
    ->getForm();

In old Symfony versions (2.0 and earlier), use property_path:

$form = $this->createFormBuilder($promo)
    ->add('code', 'text')
    ->add('image', 'file', array(
                "property_path" => false,
            ))
    ->getForm();

"property_path" was removed in Symfony 2.3