gistfile1.txt

if($this->getObject()->isNew() || $this->getValue('attachment_name') != $this->getObject()->getAttachmentName())
{
     //check if attachment filename field or attachment file field are both filled, they both have to be filled if you want to upload an attachemnt file
     $this->mergePostValidator(new AttachementNameAndAttachmentFileValidatorSchema());
}