Parse json part of a multipart request

I have a route where I need to receive an uploaded image and a json payload, to do that, I’m using multipart/form-data.

The issue is that when I receive the payload in my controller, the json part is not decoded properly, I receive it in string form.

Here is my curl request, as you can see, I am passing the corret content-type for the json payload:

curl -X POST http://localhost:4000/api/qr_code/simple \
         -H "Content-Type:multipart/form-data" \
         -F "file=@logo.svg;type=image/svg+xml" \
         -F "payload={\"my\":\"json\"};type=application/json"

The only way I managed to achieve that was to create a plug myself that do the decode:

defmodule MyParser do
  @moduledoc false
  
  @behaviour Plug

  def init(opts) do
    key = opts |> Keyword.fetch!(:key) |> to_string()

    key
  end

  def call(%{req_headers: req_headers} = conn, key) do
    with {"content-type", content_type} <- List.keyfind(req_headers, "content-type", 0),
         true <- String.starts_with?(content_type, "multipart/form-data") do
      maybe_parse_key_content(conn, key)
    else
      _ -> conn 
    end
  end

  defp maybe_parse_key_content(%{body_params: body_params} = conn, key) do
    with {:ok, content} when is_binary(content) <- Map.fetch(body_params, key),
         {:ok, decoded_content} <- Jason.decode(content) do
      put_in(conn, [Access.key(:body_params), key], decoded_content)
    else
      _ -> conn
    end
  end
end

It works, but I feel that Phoenix should do that automatically.

So, is there some way to tell the Phoenix Controller to properly parse that field for me?