here is my post man request.
I don't think the API accepts raw JSON.
I use this method,best to set Category items to not required for testing. Enter this into Body Section, as shown in image, change details to match your category, { "name": "REST API Test Word.jpg", "type": 144, "parent_id": 5645848, "roles": { "categories": { "5646542": { "5646542_2": "Europe", "5506675_4": null, "5506675_5": null, "5506675_6": null, "5506675_7": null, "5506675_8": null, "5506675_9": null, "5646542_1": { "version_number": 1 } } }, "classifications": { "create_id": [], "id": [] } } }