I have been trying to make a Form Collection with file upload in Symfony2 and following this guide
http://symfony.com/doc/master/cookbook/form/form_collections.html
When you add by_reference => false you are suposed to set the counter entity in the add function.
So your property_id should be persisting correctly in the image changing the addImage function in your propery to:
public function addImage(\Mata\MainBundle\Entity\Image $image)
{
$this->images[] = $image;
$image->setProperty($this);
return $this;
}