In the Collector App, we are unable to view / download attachments from a feature service that were uploaded either via a portal webapp (WAB) or the portal map viewer.
We can only view attachments created in Collector itself.
However, Attachments created in collector can be viewed in the portal webapp (WAB) , the portal map viewer as well as collector itself.
Portal for ArcGIS 10.5
ArcGIS Server 10.5
Oracle Enterprise GDB 12c
Collector for ArcGIS (Windows) v 17.0.3