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>