symfony - Sonata Media: The file could not be found

Question

I'm trying to attach an image to an Entity but when I submit the form, an error is displayed saying: The file could not be found.

Here is my code to build the form:

$builder
    ->add('email', 'email', array('label' => 'Email'))
    ->add('name', 'text', array('label' => 'Name'))
    ->add('image', 'sonata_media_type', array('label' => 'Image', 'provider' => 'sonata.media.provider.image', 'context'  => 'default'))
;

What am I doing wrong?

Thanks