[JAVA] Serialization of parts of multipart request bodies

Description

I am using the Swagger Codegen to generate a JAVA client to request a Spring Boot API.
This API provides a POST web service which takes a JSON parameter "dto" as a part of multipart/form-data.
I am using the "encoding" attribute in the swagger declaration but the client does not seem to use it.
The parameter is not serialized to JSON but is sent as the string representation of the class.

Swagger-codegen version

swagger-codegen-cli-3.0.5-20190130.172841-36.jar

Swagger declaration file content or url

Open API v3.0.2

"requestBody": {
  "content": {
    "multipart/form-data": {
      "schema": {
        "type": "object",
        "properties": {
          "dto": {
            "$ref": "#/components/schemas/PushConfigurationDto"
          }
        }
      },
      "encoding": {
        "dto": {
          "contentType": "application/json"
        }
      }
    }
  }
}

This case corresponds to the "Complex Serialization in Form Data" example described into this documentation.

Command line used for generation

java -jar swagger-codegen-cli-3.0.5-20190130.172841-36.jar generate -i http://192.168.5.159:8080/openApi -l java -o ./java_api_client

Suggest a fix/enhancement

Is there a solution to make the client serializing multiparts other than String?

Start with the supplied OpenAPI 3.0.2 multipart/form-data declaration and the Java generation command, then inspect the generated client's multipart parameter handling. Confirm how the dto part is serialized and verify that it is sent as application/json rather than the class's string representation.