The GP Support team has more information on the improved Document Attach Features for Microsoft Dynamics GP 2013 Service Pack 2!
Document Attach Features for Microsoft Dynamics GP 2013 Service Pack 2! – Dynamics GP Support and Services Blog – Microsoft Dynamics GP – Microsoft Dynamics Community
Microsoft & the GP EcosystemInstall, Upgrade & AdminGP 2013
Archived post from 2013. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.