Attachment Category for XML Transactions

Hi,

We have followed setup describes in

setting up XML category, which is working fine for all attachments that are attached in the invoice screen. however when searching an existing transaction and then selecting credit balance and then attaching documents, they are not picked up. There only is a section like

     <cac:AdditionalDocumentReference>

           <cbc:ID>UCMFA10417128</cbc:ID>

           <cac:Attachment>

              <cbc:EmbeddedDocumentBinaryObject uri="UCMFA10417128" filename="factuur dat gecrediteerd werd.pdf" mimeCode="application/pdf"/>

              <cac:ExternalReference>

                 <cbc:URI>FILE</cbc:URI>

              </cac:ExternalReference>

           </cac:Attachment>

        </cac:AdditionalDocumentReference

but no base64 content. Although the document is valid. Is there anything specially needed for this type of transaction so they are picked up?