non-working constraint

constraint.php

public function buildForm(FormBuilderInterface $builder, array $options) {
        $subscriber = new AddDateFieldSubscriber($builder->getFormFactory());
        $builder->addEventSubscriber($subscriber);
        $builder
            ->add('images', 'collection', array(
                'type'           => new UploadType(),
                'required'       => true,
                'allow_add'      => true,
                'allow_delete'   => true,
                'by_reference'   => false,
                'prototype'      => true,
                'widget_add_btn' => array(
                    'icon'  => 'plus-sign',
                    'label' => 'add',
                ),
                'widget_type'  => 'inline',
                'options'        => array(
                    'label_render' => false,
                    'widget_control_group' => false,
                    'widget_remove_btn' => array(
                        'icon'  => 'minus-sign',
                        'label' => 'remove',
                    )
                ),
                'constraints' => array(
                    new Collection(array(
                        'fields' => array(
                            'file' => new Image()
                        ),
                        'allowMissingFields' => true,
                    )),
                ),
            ));
    }