bug: Casting json part of a multipart/form-data fails

[sezaru]

I have an API that uses multipart/form-data, it will receive an image binary and a payload in json.

Here is how the API is defined:

OpenApiSpex.schema(%{
  type: :object,
  properties: %{
    image: %Schema{type: :string, format: :binary},
    payload: %Schema{
      type: :object,
      properties: %{
        ...
      }
    }
  }
})

And there is the controller spec:

operation :simple,
  summary: "Something",
  request_body:
    {"Params", "multipart/form-data", __MODULE__.MySchema, required: true},
  responses: [
    ok: ...
  ]

I also have cast and validation enabled in my controller:

plug OpenApiSpex.Plug.CastAndValidate, json_render_error_v2: true

Then, I try to send a request via CURL:

curl -X POST http://localhost:4000/api/qr_code/simple \
  -H "Accept: application/json" \
  -F "file=@logo.svg;type=image/svg+xml" \
  -F 'payload={
    "userId": "12345",
    "description": "Example SVG upload",
    "tags": ["icon", "vector"]
  }'

NOTE: I also tried the same request with Req, giving the same reply.

What I expected is to receive in my controller is a struct containig both the image in a Plug.Upload struct and the payload in json format.

But calling that API will return the following error:

%{
  "errors" => [
    %{
      "detail" => "Invalid object. Got: string",
      "source" => %{"pointer" => "/payload"},
      "title" => "Invalid value"
    }
  ]
}

Which indicates that the payload value is being returned as a string, not as a decoded json.

I disabled the cast and validation plug in my controller and sure enough, this is what I'm receiving:

%{
  "file" => %Plug.Upload{
    path: "/tmp/plug-1684-gCHg/multipart-1765843491-22636011688-4",
    content_type: "image/svg+xml",
    filename: "logo.svg"
  },
  "payload" => "{\n    \"userId\": \"12345\",\n    \"description\": \"Example SVG upload\",\n    \"tags\": [\"icon\", \"vector\"]\n  }"
}

So, my question is, shouldn't OpenApiSpex cast identify that the payload in this case is a string and decode it using JSON?

Is this a bug or an I doing something wrong with my schema?