YAML request body rendered as JSON

Q&A (please complete the following information)

• OS: macOS 10.13.6
• Browser: Chrome
• Version: 69.0.3451.0
• Method of installation: docker image
• Swagger-UI version: 3.17.0
• Swagger/OpenAPI version: OpenAPI 3.0

Content & configuration

Swagger/OpenAPI definition:

openapi: 3.0.0
info:
  title: Yet Another REST API
servers:
  - url: 'https://example.com/api'
components:
  securitySchemes:
    basicAuth:
      type: http
      scheme: basic
  schemas:
    Pool:
      type: string
      example: dev
    ServiceType:
      type: string
    StackName:
      type: string
      pattern: '^[a-zA-Z0-9-]+$'
      example: jd-stack
    StackNew:
      type: object
      properties:
        owner:
          $ref: '#/components/schemas/UserID'
        stackname:
          $ref: '#/components/schemas/StackName'
        version:
          $ref: '#/components/schemas/StackVersion'
        services:
          type: array
          items:
            type: object
            properties:
              type:
                $ref: '#/components/schemas/ServiceType'
              servers:
                type: integer
                minimum: 1
                example: 1
        poolname:
          $ref: '#/components/schema/Pool'
    StackVersion:
      type: integer
      example: 6
    UserID:
      type: string
      format: email
      example: john.doe@example.com
security:
  - basicAuth: []
paths:
  /stacks:
    post:
      description: Creates new stack
      tags:
        - Stacks
      requestBody:
        required: true
        content:
          application/yaml:
            schema:
              $ref: '#/components/schemas/StackNew'
      responses:
        '201':
          description: Created
          content:
            application/json:
              schema:
                type: object
                properties:
                  stackname:
                    $ref: '#/components/schemas/StackName'

Swagger-UI configuration options:
Defaults on docker image

Screenshots

How can we help?

Not sure if this is the expected behaviour, but I'd expect a request body of media type application/yaml to be rendered as yaml instead of json. Any help/advice on this would be much appreciated!

Reproduce the supplied OpenAPI 3.0 definition in Swagger UI 3.17.0, focusing on the /stacks POST request body with application/yaml content. Trace how the request-body media type is rendered and verify that the completed behavior displays YAML rather than JSON.