Question
How to grant rights to the specific email using business process?
Answer
To grant access rights to a specific email you need to open it and go to Actions -> Set up access rights and add the necessary users there:
Once the emails are visible to the users, the attachments will be too.
If you need all the emails attached to cases be visible for the users by default, you can create a business process that will start every time the new activity with type = Email and CaseId filled in is added. The Change Access Rights business process element should be used to grant the rights for all the users that see the case. To find the correct users you can use Read Data element and read the users connected to the case. In case you'd like to grant the rights for all employees, it's even better. Just put 'All Employees' in Change Access Rights process element.