Uploads an image for an order's content.

Send multipart/form-data with file, an image of at most 5 MB and at least 100×100 px, and order, the id of the order it belongs to. The client uploads for its own orders, the publisher for orders it works on. The answer's @id is /api/media_objects/{id}. To show the image in the article, put <img src="https://<API host>/api/media_objects/{id}"> with the full URL into contentRevision.body (PATCH /api/orders/{id}/update-content). A wrong file or an unknown order is answered 422 with the reason.