The hits keep coming. This is a popular request from client I work with. Microsoft Dynamics GP 2015 R2 New Features – Workflow Document Attachment
Microsoft Dynamics GP 2015 R2 New Features – Workflow Document Attachment – Microsoft Dynamics GP Community
Tips & Hidden FeaturesInstall, Upgrade & AdminGP 2015
Archived post from 2015. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.