let’s say you want to constraint what people are uploading and you don’t want to use assert statements in the entity. you can do it in the formbuilder
use Symfony\Component\Validator\Constraints\File; ... $form = $this->createFormBuilder($entity) ->add('logo', FileType::class, array( 'label' => false, 'attr' => array( 'accept' => "image/jpeg, image/png" ), 'constraints' => [ new File([ 'maxSize' => '2M', 'mimeTypes' => [ 'image/jpeg', 'image/png', ], 'mimeTypesMessage' => 'Please upload a JPG or PNG', ]) ] ) )