[Spring] Generated server code for multipart/form-data request ignores encoding section.

Description

As described in https://swagger.io/docs/specification/describing-request-body/multipart-requests, it is possible in OpenAPI 3.0.0 to specify for a multipart request an encoding section for example to enforce contentType or the presence of a specific header in the MIME part. However the server-side code generation seems to completely ignore this section.

Swagger-codegen version

3.0.26

Swagger declaration file content or url

openapi: 3.0.0
info:  
  version: "0.1"
  title: testbug
paths:
  /testbug:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                attachment:
                  type: string
                  format: binary
            encoding:
              attachment:
                contentType: application/pdf
                headers:
                  Content-ID:
                    schema:
                      type: string
      responses:
        '201':
          description: Successful response.
          content:
            text/plain:
              schema:
                type: string

Command line used for generation

java -jar .\swagger-codegen-cli-3.0.26.jar generate -l spring -i testbug.yaml -o testbug

Steps to reproduce

java -jar .\swagger-codegen-cli-3.0.26.jar generate -l spring -i testbug.yaml -o testbug

In the generated server-side code I find nowhere a constraint on contentType = application/pdf and neither on the presence of a header Content-ID nor a way to retrieve the Content-ID using the generated Java code.

The issue was tested with multipart/form-data but also with multipart/related (having a way to retrieve Content-ID would mainly be interesting for us when using multipart/related).

Related issues/PRs

None.

Suggest a fix/enhancement

None.

Start by running the reported Spring generation command with the provided OpenAPI document and inspect the generated server-side code for the multipart request. Trace how the multipart/form-data and multipart/related encoding sections are handled during generation. Done means the generated Java server code reflects the declared part content type and provides a way to access the Content-ID header.