Your sigh is imprecise in two details:
- it is not accessible from portal GUI
- it is not accessible if you use attachments added via Add attachment (utilizes the ZipRenditions component)
ZipRenditions' attachments are auxiliary files attached to the main content item. They have no metadata, cannot be searched for, but they might have some information value added to the main item - imagine a use case, when a contract is the main item, and a scanned ID card is added to it.There are other means how to create an attachment - see this thread: Re: How to push bunch of content items together in workflow
However, this is all WCC-side. Portal has its own linking, invisible to WCC. If you mean WebCenter Portal, in there you can create a link not only between documents, but between any two objects (announcements, discussions, documents, tasks, ...). On the other hand, some WCC features, such as ZipRenditions' attachments might not be available in the standard GUI (you could use them, but this would require customization of portal's task flows to expose more WCC's services).
Therefore, I'd recommend to change your implementation design: create both the main item and its attachments as "normal items" and link them using the portal's links.
You could get the file either using the following URL format or by calling the GET_FILE service using the &Rendition parameter
URL - http://<Host Name>:16200/cs/idcplg?IdcService=GET_FILE&dID=1111&dDocName=TESTDOCUMENT&Rendition=<Rendition Name of The Attachement File>&allowInterrupt=1