remove option for file upload field

Acme\DemoBundle\Form\EventListener\AddRemoveImageFieldSubscriber.php

<?php

namespace Acme\DemoBundle\Form\EventListener;

use Symfony\Component\Form\Event\DataEvent;
use Symfony\Component\Form\FormFactoryInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Form\FormEvents;

class AddRemoveImageFieldSubscriber implements EventSubscriberInterface
{
  private $factory;

	public function __construct(FormFactoryInterface $factory)
	{
		$this->factory = $factory;
	}

	public static function getSubscribedEvents()
	{
		// Tells the dispatcher that we want to listen on the form.pre_set_data
		// event and that the preSetData method should be called.
		return array(FormEvents::PRE_SET_DATA => 'preSetData');
	}

	public function preSetData(DataEvent $event)
	{
		$data = $event->getData();
		$form = $event->getForm();

		// During form creation setData() is called with null as an argument
		// by the FormBuilder constructor. We're only concerned with when
		// setData is called with an actual Entity object in it (whether new,
		// or fetched with Doctrine). This if statement let's us skip right
		// over the null condition.
		if (null === $data) {
			return;
		}

		// check if the data object has an image
		if ($data->getImage()) {
			$form->add($this->factory->createNamed('checkbox', 'removeFile', null, array('required' => false)));
		}
	}

}

gistfile1.twig

{{ form_label(form.file,'form.label_file'|trans({}, admin.translationDomain)) }}
<div class="input">
    <ul class="inputs-list">
    {% if form.vars.data.image %}
        <li><img src="{{ asset(form.vars.data.webPath) }}" /></li>
        <li>
            <label for="{{ form.removeFile.get('id') }}">
                {{ form_widget(form.removeFile) }}
                <span>{{ 'form.label_remove_file'|trans({}, admin.translationDomain) }}</span>
            </label>
        </li>
    {% endif %}
        <li>
            {{ form_widget(form.file) }}
            {{ form_errors(form.file) }}
        </li>
    </ul>
</div>

gistfile2.php

// ...
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            // ...
            ->add('file', null, array('required' => false))
        ;

        // only add a remove file checkbock when the document has an image
        $subscriber = new AddRemoveFileFieldSubscriber($builder->getFormFactory());
        $builder->addEventSubscriber($subscriber);
    }
// ...