/vendor/magento/framework/Config/etc/view.xsd

<xs:attribute name="type">
                        <xs:simpleType>
                            <xs:restriction base="xs:string">
                                <xs:enumeration value="thumbnail"/>
                                <xs:enumeration value="small_image"/>
                                <xs:enumeration value="image"/>
                                <xs:enumeration value="swatch_image"/>
                                <xs:enumeration value="swatch_thumb"/>
                                <xs:enumeration value="image_productlist"/>
                                <xs:enumeration value="image_produktlist_hover"/>
                            </xs:restriction>
                        </xs:simpleType>
                    </xs:attribute>