The relevant documents attribute for dictionary document items (and therefore links to attachments) are not available in the collaboration hub, for normal hub users. However, they are available in collaboration hub in preview mode, for administrators and process modelers.
Publish the document in explorer. For the relevant document attribute and the link to the attachment to be visible in the collaboration hub, the document needs to be published.
KBA , BPI-SIG-PM-DIC , Dictionary for SAP Signavio Process Manager , How To
SAP Process Collaboration Hub by Signavio all versions