We have an external system to store all the Documents, for which we have REST connectors in place to store the documents to external system and retrieve them.

To keep it generic we have used the CMIS rules and customized them to store the documents to external source instead of Content Management which Pega allows configuration for.

While saving the attachment for WO W1 with class A-B-C lets say an Attachment Category A1 is created in the class A-B-C and used in the W1 , there is a functionality which creates a linked case in class A-B-D and the attachment belonging to attachment category A1 gets copied to the linked case.

When we try to open the attachment belonging to A1 in the WO class A-B-D , its not opening the documents which are supposed to open inline (e.g. PDF / JPEG) the ones which can be downloaded like doc, xlsx are getting downloaded.

Also, this is not happening when the documents are stored in Pega in the above scenario, only when we try to retrieve the documents stored in external system has this problem

Is there something which Pega is invoking OOTB to view these attachments which I need to make sure is included while retrieving external source documents?
Any other help would be appreciated.