Attachments attached to the catalog item or record producer appear on the RITM record but not on the sc_task record even though there is a business rule to process this. Whilst this works fine inside the Service Catalog, this doesn't seem to work in the Service Portal.
Kingston release and onwards
Whilst this is related to a problem that was fixed in Kingston, the issue is actually caused by a deprecated SC Catalog Item widget being used.
A new widget was released from Kingston onwards, where the old one was renamed to have "Deprecated" in the front of it.
To determine if you are using the older widget, press CTRL+right-click on the widget to see the name, it should look similar to the below image:
The resolution to this issue is that you need to modify the Service Portal page and remove the old widget whilst replacing it with the new one released.