This happens because the “Append attachment contents” feature relies on Task/Activity associations, not just general object relationships. Only objects that support creating Tasks (i.e., have a Task surrogate) appear in the dropdown.
Details / Explanation:
The Append Attachment Contents function is tied to Salesforce’s activity model.
Only objects with “Allow Activities” enabled can be used for appending attachments.
If your custom object doesn’t have this setting, it won’t appear in the list.
Resolution / Steps:
Go to Salesforce Setup → Object Manager → [Your Custom Object] → Edit.
Enable the checkbox “Allow Activities.”
Save your changes.
This creates the necessary Task surrogate for the object. Once done, the object will appear in the “Append attachment contents” dropdown, allowing you to include related file attachments in your merge action.