I'm afraid this is a bit of a limitation in the product. You can send multiple attachments using the multiple attachment attribute, however you will still only be able to pick up value from one attribute using runtime values. So if your attachments are all added separately in to different collections and such different attribute, you will not be able to automatically combined them into the one attachment attribute on the reminder.
I would suggest that you add an ER on our Community to suggest that inbound emails should attach all its emails to the one attachment collection and attachment attribute if the mapped attachment attribute is a multiple attachment type. You can see our Enhancement Request Portal here: https://community.landesk.com/support/community/enhancementrequests