Responses schema from @ApiOperation is not picked up

[tiholic]

I'm submitting a...

[ ] Regression 
[x] Bug report
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

I want to create responses schema for returning file content.

The schema should be something as described here: https://swagger.io/docs/specification/describing-responses/#response-that-returns-a-file

Instead this gets returned from -json endpoint

{
  "/api/v2/download/": {
    "get": {
      "operationId": "HELO",
      "summary": "",
      "responses": {
        "200": {
          "description": ""
        }
      },
      "parameters": [],
      "tags": [
        "v2api"
      ],
      "security": [
        {
          "jwt": []
        }
      ]
    }
  }
}

Expected behavior

Expect responses to show schema as configured (see below snippet)

Minimal reproduction of the problem with instructions

Source code to replicate the issue:

@ApiBearerAuth('jwt')
@ApiTags('v2api')
@ApiExtraModels(
 Dto1, Dto2     // using for other purposes
)
export class APIController {

  // some other API controller methods

  @ApiOperation({
    operationId: 'HELO',
    responses: {
      '200': {
        description: 'download',
        content: {
          'application/pdf': {
            schema: {
              type: 'string',
              format: 'binary',
            },
          },
          'application/png': {
            schema: {
              type: 'string',
              format: 'binary',
            },
          },
        },
      },
    },
  })
  @Get('/download/')
  helo() {
    // TODO download
  }
}

Environment

Nest version: X.Y.Z
```
        "@nestjs/common": "7.4.2",
        "@nestjs/core": "7.4.2",
        "@nestjs/jwt": "7.1.0",
        "@nestjs/passport": "7.1.0",
        "@nestjs/platform-express": "7.4.2",
        "@nestjs/swagger": "4.8.1",
        "@nestjs/cli": "7.4.1"
```
 
For Tooling issues:
- Node version: v12.10.0
- Platform:  Mac

[kamilmysliwiec]

Would you like to create a PR for this issue?

[tiholic]

I haven't looked at the source yet!, But happy to contribute if you can point me where to look for the issue?

[kamilmysliwiec]

Side question, any reason you don't want to use the @ApiResponse() decorator instead?

[tiholic]

I want to configure multiple response content types. Download could be a PDF or Image or some other document type (document types are limited)

[kamilmysliwiec]

Let's track this here #1454