call.json

  {
    "data": {
      "relationships": {
        "sender": {
          "data": {
            "type": "account",
            "id": "70960946-cc83-4ce1-8071-03ed72ffb07e"
          }
        },
        "files": [
          {
            "data": {
              "type": "file",
              "id": "653d2888865f99c885cbbe2e9170fee6b4f1761d815b7f1203a2c86cb0550ad2"
            }
          }
        ],
        "recipient": {
          "data": {
            "type": "account",
            "id": "bbc57904-3c09-4f87-b9be-e60a1952c81a"
          }
        }
      }
    }
  }

result.json

  {
    "errors": [
      {
        "status": 400,
        "title": "bad-request",
        "code": "bad-request",
        "detail": "You must specify a type of file or a set of files to transfer.",
        "source": {
          "pointer": "/data/relationships/file-type/data/id"
        }
      }
    ]
  }