Charset of text/plain response is ignored

Q&A (please complete the following information)

• Swagger-UI version: 5.17.14
• Swagger/OpenAPI version: OpenAPI 3.0

Content & configuration

Example Swagger/OpenAPI definition:

openapi: 3.0.1
info:
  title: Test
  version: 1.0.0
servers:
- url: http://localhost:8080
paths:
  /test:
    get:
      responses:
        "200":
          description: OK

Swagger-UI configuration options:

SwaggerUIBundle({
  url: "https://petstore.swagger.io/v2/swagger.json",
  dom_id: '#swagger-ui',
  deepLinking: true,
  presets: [
    SwaggerUIBundle.presets.apis,
    SwaggerUIStandalonePreset
  ],
  plugins: [
    SwaggerUIBundle.plugins.DownloadUrl
  ],
  layout: "StandaloneLayout",
  configUrl: "/v3/api-docs/swagger-config",
  operationsSorter: "alpha",
  tagsSorter: "alpha",
  validatorUrl: ""
})

Describe the bug you're encountering

To reproduce...

Steps to reproduce the behavior:

1. Create a webserver listening on /test and returning text/plain;charset=X with a charset other than UTF-8.
2. Send a request via Swagger UI.
3. In the response body view, the response appears to be decoded with UTF-8.

Expected behavior

I expected the response to be decoded with the charset provided in the Content-Type header.

Additional context or thoughts

Works in other clients like Chrome DevTools.

Start by reproducing the issue in Swagger UI with a webserver returning text/plain;charset=X for a non-UTF-8 charset, then trace the response body view's handling of the Content-Type header. Done means the response body is decoded using the charset provided by the header rather than always as UTF-8.