You can extend an attribute group for linking to add a content type:
In types.xsd:
<xs:attributeGroup name="linking"> <xs:attribute name="href" type="xs:anyURI" use="required"/> <xs:attribute name="alt" type="xs:string"/> <xs:attribute name="title" type="xs:string"/> </xs:attributeGroup>
The redefine:
<xs:redefine schemaLocation="types.xsd"> <xs:attributeGroup name="linking"> <xs:attributeGroup ref="my:linking"/> <xs:attribute name="content-type" type="xs:string"/> </xs:attributeGroup> </xs:redefine>