Attaching Files Uploaded from a Nintex Form to the Appropriate SharePoint List item

I am new to Nintex Workflow Development. 

I have created a couple of forms and associated workflows using Nintex Workflow Cloud.  The form is using the "File Upload" control. Files appear to upload to the form via the "File Upload" control. 

I am using the SharePoint Online "Add Attachment to an Item: control within my Nintex workflow. 

The goal is to take the files uploaded via the form and save them as attachments to a SharePoint Online List item.

However, I can't seem to figure out how to get it to work.

When I try using the "Add Attachment to List Item" control in my workflow, I get the following error.

Add attachment to an item (Waiting)
Failed 
Received an error response from the connector: Json file source is empty.. [Error Code: IO.XtensionExecute.ErrorResponse]

Is there a user guide, tutorial, video, or someone with experience that can show/tell me how to get the file(s) from a form into a SharePoint list item as an attachment via a Nintex workflow?

Thank you in advance for your help.