mimetype-map-custom.xml

<?xml version="1.0" encoding="UTF-8"?>
<alfresco-config area="mimetype-map">
    <config evaluator="string-compare" condition="Mimetype Map">
        <mimetypes>
            <mimetype mimetype="application/x-test" display="test_ext">
                <extension>test</extension>
            </mimetype>
        </mimetypes>
    </config>
</alfresco-config>