This is the closest question (no answer) I have found: https://community.nintex.com/t5/Nintex-for-SharePoint-Forum/Can-Nintex-Workflow-using-Web-Request-Ac...
I need to upload a file to a web service using the JSON action. The POST REST call requires multipart/form-data. I am not sure how to do this or whether it is possible with the JSON action.
Solved! Go to Solution.
You may need to create your own action to deal with this.
In the nintex help on Xtensions there is a guide on multipart file handling.
Apologies I'm not a dev so I haven't tried creating my own Xtension as yet but hopefully this may lead you to a solution.