You can add attachments to any process variable, as well as manipulate attachments with a set of functions, described in
Custom Functions for Manipulating Attachments.
Currently, to view attachments in Process Developer, you can add them as a simulation property, described in
Adding an Attachment for Simulation.
Be sure to initialize a process variable, if it has not been initialized, before copying attachments to it. See
Initializing a Variable for details.