Default `Content-Type` not applied to complex object parts in `multipart/form-data`

Q&A

• OS: macOS
• Browser: Firefox
• Version: 122
• Method of installation: swagger-ui-dist from unpkg
• Swagger-UI version: 5.11.2
• Swagger/OpenAPI version: OpenAPI 3.0.3

Content & configuration

Example Swagger/OpenAPI definition:

openapi: 3.0.3
servers:
- url: http://127.0.0.1:8080
paths:
  /profile:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              required:
              - address
              - id
              - profileImage
              type: object
              properties:
                id:
                  type: string
                address:
                  type: object
                  properties:
                    street:
                      type: string
                    city:
                      type: string
                profileImage:
                  type: string
                  format: binary
      responses:
        "200":
          description: OK

Swagger-UI configuration options:

SwaggerUIBundle({
  url: 'http://127.0.0.1:8080/api-docs.yaml',
  dom_id: '#swagger-ui',
  presets: [
    SwaggerUIBundle.presets.apis,
    SwaggerUIStandalonePreset
  ],
  layout: "StandaloneLayout",
})

Describe the bug you're encountering

According to the OpenAPI specification and the docs page, the default Content-Type for complex values (objects) in a multipart/form-data request should be application/json.
However, in the provided example, the address property, which is an object, does not have application/json as its Content-Type in the generated request and curl command.

Currently, it's possible to manually set encoding.address.contentType in the definition as application/json to generate the expected curl command (thanks to issue #5356 was fixed).
However, based on the OpenAPI specification, I believe type=application/json should automatically be applied even if encoding is not explicitly included in the definition.

To reproduce...

Steps to reproduce the behavior:

1. Fill the blanks for the /profile request
2. Click on 'Execute'
3. Generated curl command and payload are:

curl -X 'POST' \
  'http://127.0.0.1:8080/profile' \
  -H 'accept: */*' \
  -H 'Content-Type: multipart/form-data' \
  -F 'id=string' \
  -F 'address={
  "street": "string",
  "city": "string"
}' \
  -F 'profileImage=@zerobyte.txt;type=text/plain'

-----------------------------306494087715324971512255078872
Content-Disposition: form-data; name="id"

string
-----------------------------306494087715324971512255078872
Content-Disposition: form-data; name="address"

{
  "street": "string",
  "city": "string"
}
-----------------------------306494087715324971512255078872
Content-Disposition: form-data; name="profileImage"; filename="zerobyte.txt"
Content-Type: text/plain


-----------------------------306494087715324971512255078872--

Expected behavior

 curl -X 'POST' \
   'http://127.0.0.1:8080/profile' \
   -H 'accept: */*' \
   -H 'Content-Type: multipart/form-data' \
   -F 'id=string' \
   -F 'address={
   "street": "string",
   "city": "string"
-}' \
+};type=application/json' \
   -F 'profileImage=@zerobyte.txt;type=text/plain'

 -----------------------------306494087715324971512255078872
 Content-Disposition: form-data; name="id"
 
 string
 -----------------------------306494087715324971512255078872
 Content-Disposition: form-data; name="address"
+Content-Type: application/json
 
 {
   "street": "string",
   "city": "string"
 }
 -----------------------------306494087715324971512255078872
 Content-Disposition: form-data; name="profileImage"; filename="zerobyte.txt"
 Content-Type: text/plain
 
 
 -----------------------------306494087715324971512255078872--

Additional context or thoughts

• I believe specifying Content-Type: text/plain for the id property is unnecessary, as it is the default value for multipart/form-data.
• This can be swagger-js issue.

EDIT: this is part (maybe duplicate) of #6462

Start with the SwaggerUIBundle reproduction using the supplied OpenAPI 3.0.3 multipart example, then trace request generation for the address object and its generated curl command. Done means complex object parts include an application/json Content-Type while simple fields retain their expected defaults.