如何允许Cakephp 3验证的文件类型为空

Hi I'm trying to do a validation for a file field on Cakephp3. This is what I have so far.

$validator
  ->add('file_upload', 'custom', [
     'rule' => function ($value, $context) {
      return empty($context['data']['file_type']);
      },
    'message' => 'Cannot be empty if file type is filled out'
  ]);

I originally had allow empty for this field but realized that the file types comes in as an array and wouldn't validate properly, so tried to used a custom validator and still doesn't seem to work. Comments and critiques are welcomed.