You can add case wide local action (AddNote) to give users the ability to add notes (stored in pyNotes property) which gets added to the Attachments gadget.
For example within Smart Dispute app , you can PegaCard-Sd-Dispute-.AddNote flow action and you can see the notes attached in the Attachment section and also Audit updated with the note attached.
Smart Dispute is using the Pega Platform OOTB Work-.AttachNote activity, not much of documentation is available around this as it’s one of legacy Attachment category.