ezpublish:
    system:
        my_siteaccess:
            field_templates:
                - 
                    template: "AcmeTestBundle:fields:my_field_template.html.twig"
                    # Priority is optional (default is 0). The higher it is, the higher your template gets in the list.
                    priority: 10

You can define these rules in a dedicated file instead of ezpublishapp/config/ezpublishezplatform.yml. Read the cookbook recipe to learn more about it.