Symptoms

A Sharepoint 2013 List Attachment Smart Object (SMO) will not show an associated “Item Reference” in the context browser of workflow steps if it was added after the project initially had K2 applied to the List. Regenerating the Smart Object and Forms/Views after adding the SMO does not cause it to appear either.
 

Diagnoses

Reproduced on two VMs with 4.6.11 installed. Deleting and then adding the workflow again will add an item reference.
 

Resolution

Attachments should be only disabled on the list if you are 100% sure that you will not need to use an item reference to the attachments smart object in your workflow. Also deleting the workflow that existed when the list was appified with attachments disabled and then creating the workflow again after enabling attachments and regenerating smart objects will add an item reference in the context browsers. This is obviously not the best option because you would have to redo an entire workflow but it appears to be the only current workaround to solve this issue.