The accepted answer works if you set the JSON as a key/value pair in the form-data
panel (See the image hereunder)
Nevertheless, I am wondering if it is a very clean way to design an API. If it is mandatory for you to upload both image and JSON in a single call maybe it is ok but if you could separate the routes (one for image uploading, the other for JSON body with a proper content-type header), it seems better.