Before merging duplicate cases together, you're viewing the case (which will eventually be selected as the Child) and there is an Email Message record which includes an attached File. That File displays in the Files related list.

 

Upon merging, the Email Message and the attached File are successfully merged to the Parent Case. However, the File is not accessible from the Files related list.
 

In the Winter 19 release, Salesforce introduced a new setting for Email-to-Case, "Save Email-to-Case attachments as Salesforce Files." When enabled, inbound Email Message record including attachments will save those attachments under the Files object (instead of the Attachments object.)

During inbound email processing, the "Has Attachments" field on Email Message records is populated if the incoming email includes an attachment.

During the merge process, Email Message records and their respective attachments are cloned to the Parent Case. In the event the original attachment was saved as a File in Salesforce (not an Attachment), the "Has Attachments" field was not checked on the cloned Email Message records.




The Email Message attachments (being saved as Files) are still successfully cloned with the Email Message onto the new case. However, because the "Has Attachments" checkbox was not checked, the Files related list in Lightning (viewed from the record detail page) does not display these images.

The "Has Attachments" field is NOT directly accessible by developers.

After discussing this limitation with Salesforce Support, it was learned that setting the "IsClientManaged" field on Email Message allowed for the "Has Attachments" field to be successfully populated. Versions 3.71 and newer of Case Merge Premium include this updated logic.