[BUG][GO] go-server generator doesn't generate endpoint for file download, it's always JSON

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

I want to create an endpoint which allows downloading files based on some parameters. I followed the official instructions
on how to describe this kind of response: https://swagger.io/docs/specification/describing-responses/
But no matter what I try the generated endpoint returns JSON.

openapi-generator version

openapitools/openapi-generator-cli:latest

OpenAPI declaration file content or url

...
/v1/address/postcode-ranges:download:
  post:
    requestBody:
      content:
        application/json:
          schema:
            $ref: './schema.yaml#/V1DownloadPostcodeRangesFileRequest'
    responses:
      "200":
        description: Excel file
        content:
          application/octet-stream:
            schema:
              type: string
              format: binary

Generation Details

func (c *AddressApiController) V1AddressPostcodeRangesdownloadPost(w http.ResponseWriter, r *http.Request) {
	v1DownloadPostcodeRangesFileRequestParam := V1DownloadPostcodeRangesFileRequest{}
	d := json.NewDecoder(r.Body)
	d.DisallowUnknownFields()
	if err := d.Decode(&v1DownloadPostcodeRangesFileRequestParam); err != nil {
		c.errorHandler(w, r, &ParsingError{Err: err}, nil)
		return
	}
	if err := AssertV1DownloadPostcodeRangesFileRequestRequired(v1DownloadPostcodeRangesFileRequestParam); err != nil {
		c.errorHandler(w, r, err, nil)
		return
	}
	result, err := c.service.V1AddressPostcodeRangesdownloadPost(r.Context(), v1DownloadPostcodeRangesFileRequestParam)
	// If an error occurred, encode the error with the status code
	if err != nil {
		c.errorHandler(w, r, err, &result)
		return
	}
	// If no error, encode the body and the result code
	EncodeJSONResponse(result.Body, &result.Code, w)

}

Steps to reproduce

	docker run $(DOCKER_USER) --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:latest generate \
		-i /local/docs.yaml \
		-g go-server \
		-o /local/_build/go-server \
		--additional-properties=onlyInterfaces=true && \
		cd ./_build/go-server/go && \
		go mod tidy && \
		go fmt ./... && \
		goimports -w .

Related issues/PRs

Suggest a fix

Start with the go-server generator and the generated AddressApiController method shown in the report, then trace how an application/octet-stream response is handled. Reproduce the generation with the provided OpenAPI declaration and command; done means the generated download endpoint does not encode the binary response as JSON.