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-Action-POST-a-multipart/m-p/38532
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.
HI @msalamon
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.
Files as multipart formdata (nintex.com)
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.
i am also facing same problem, how can i solve this
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.