You can add extra css to your field with attr :
$builder->add('file', 'fie', array('label' => 'My file', 'attr' => array('style' => 'display:none')));
or
//CSS File
.notvisibleclass{
display:'none'; }
//Form file
$builder->add('file', 'fie', array('label' => 'My file', 'attr' => array('class' => 'novisibleclass')));