Hi khalil,
The use case you are trying to build is unfortunately not possible with Workdesk (version smaller 4.2) and the legacy Activit API (Alfresco version smaller 4.2): The OW_Attachment property is no Activit property at all and only allows to attach documents and no folders resp. eFiles. This is why you are seeing an error when attaching a folder to a workflow.
This is why in the past I suggested (in the topic you linked) the solution to use kind of an efile ID as reference (NOT ATTACH THE EFILE!) and store it in the custom workflow´s properties. If a user is clicking on that ID then, a custom written field control in Workdesk could handle that click and open the related eFile as configured in owmimetable.xml then.
With Alfresco 4.2 in combination with the upcoming Workdesk 4.2 the behavior has changed and it is then possible to attach complete eFiles. See my examplary screenshots taken out of the development version, where I attached a dossier "Dennis Koch" to a Review task successfully.
Best regards,
Dennis