Web Portal attachments do not carry over to NXT, only database view at this time. This is something that is in the works for Web Portal in NXT and you are encouraged to support this expanding this functionality using the following link:
https://financialedge.ideas.aha.io/ideas/FENXT-I-25

You can add attachments originating in Web Portal as needed to the records in NXT using the steps found here: https://www.blackbaud.com/support/howto/coveo/fenxt/fe-attachments.html