I can't answer this question directly ; but I can give you some information I found out whilst looking into this - maybe this will be enough to get you started on this - or maybe somebody else can fill in the gaps.

Firstly: this article may be of use - it doesn't deal with CMIS attachments (just standard PRPC attachments) - but it might still shed some light on what needs to be done here :

https://collaborate.pega.com/question/how-create-zip-archive-and-attach-case

In fact in that link above - I previously mentioned that there are different PRPC Classes (Data-WorkAttach-CMIS for instance) used for different types of PRPC attachments: further research shows that this isn't necessarily the case (and the later prhelps miss this section off as well).

It seems that even for CMIS attachments - instances of the 'Data-WorkAttach-File' are created; here are three screenshots - showing me having uploaded an attachment to CMIS (Alfresco Community Edition - using the ATOM connector) via PRPC:

Here's the same document as viewed directly in Afresco:

And back in PRPC - we can see that an Data-WorkAttach-File instance was created for this attachment:

If we take a look at the Page Data for this instance, we can see the CMIS coordinates for this document (amongst other things):

<pzDocumentKey>DATA-WORKATTACH-FILE OBQ6DY-GCS-WORK T-2!20170104T150346.591 GMT</pzDocumentKey>
[...]
<pxAttachKey>20170104T150346.591 GMT</pxAttachKey>
[...]
<pxAttachKey>20170104T150346.591 GMT</pxAttachKey>
[...]
<pyContentLocation>ab918b7d-6a92-4005-a6d8-55e0da31f1ce;1.0</pyContentLocation>
[...]
<pxRefObjectKey>OBQ6DY-GCS-WORK T-2</pxRefObjectKey>
[...]

According to the 'View Definition' of the class DATA-WORKATTACH-FILE the keys for this Class are:

pxRefObjectKey
pxAttachKey

One to reference the work object ; and one to represent the attachment (it seems).

The 'pyContentLocation' appears to be the Alfresco / CMIS repository ID.

If this was a standard PRPC attachment; we would be able to directly download and decode (from 'base64') the content of the 'pyAttachStream' property  - but there is an added step needed here, since the content is stored in the CMIS repository.

I ran a TRACER against my Pega7 Portal: and clicked on the download attachment link.

This shows quite a few Activities being fired off - but I think one of the most interest is this one:

CMIS-ServiceData.pyGetContentStream|Pega-Content:07-10-01

I haven't worked out how to use this yet; but (rather nicely) this is well documented on the History Tab of the Activity ; I include this here for completeness:

Call the activity that will perform the CMIS getContentStream API method.
Activity Primary Page:

Call this activity using a primary page of a class that extends the CMIS-ServiceData class.

Parameters:

1. CMISEndpoint

This optional parameter is the Service Name key value of the connector rule that will be used for invoking the service. This connector rule instance should belong to the class of the primary page passed to this activity. If this value is not provided on the parameter page, it will be retrieved from CMIS configuration data associated with the user's current application.

2. RequestPage

This required parameter is the name of the page that contains the request data for the CMIS service. The class of the page is specified on the Pages & Classes tab of this activity. Certain property values on the request page are required. Check the pyValidateInputs activity in the class of the request data page to see which property values are required.

3. ResponsePage

This optional parameter is the name of the page that will contain the data returned from the CMIS service after the activity successfully completes. If not specified, the response data will be discarded.

Error Handling:

This activity throws a runtime exception in the event that the required input data is not valid, or the CMIS connector fails when invoked.

Hopefully this might help you start off here - I will try and return to this post if I manage to work out the next steps here.