[BUG][JavaSpring] Multipart request regression in 6.x

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

It seems the PR #11449 breaks with some multipart requests.

With an endpoint:

• Consuming multipart/form-data
• File as parameter
• A second parameter consisting of an object

and the generator settings:

• skipFormModel set to false to use the second parameter
• interfaceOnly set to true to only generate models & interfaces

The generated API causes an error while spring tries to convert the request part to the object:

Resolved [org.springframework.web.method.annotation.MethodArgumentConversionNotSupportedException: Failed to convert value of type 'java.lang.String' to required type 'com.example.demo.api.model.TestObjectPart'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.lang.String' to required type 'com.example.demo.api.model.TestObjectPart': no matching editors or conversion strategy found]

openapi-generator version

It's a regression introduced with openapi-generator 6.x.
With the 5.x version the generated code behaves as expected.

OpenAPI declaration file content or url

Or see the reproduction repo: https://github.com/cc-ju/openapi-genrator-spring-error-reproduction/blob/main/openapi.yaml

openapi: 3.0.2
info:
  version: 1.0.0
  title: test

paths:
  /test:
    post:
      requestBody:
        content:
          multipart/form-data:
            encoding:
              file:
                contentType: "application/octet-stream"
              content:
                contentType: "application/json"
            schema:
              type: object
              required:
                - file
                - content
              properties:
                file:
                  type: string
                  format: binary
                content:
                  $ref: "#/components/schemas/testObjectPart"
      responses:
        200:
          description: OK

components:
  schemas:
    testObjectPart:
      type: object
      required:
        - foo
      properties:
        foo:
          type: string
        bar:
          type: number

Generation Details

Actual output

    default ResponseEntity<Void> testPost(
        @Parameter(name = "file", description = "", required = true) @RequestPart(value = "file", required = true) MultipartFile file,
        @Parameter(name = "content", description = "", required = true) @Valid @RequestParam(value = "content", required = true) TestObjectPart content
    ) {
        return new ResponseEntity<>(HttpStatus.NOT_IMPLEMENTED);

    }

Expected output

Note that this would re-introduce the problem #11449 tried to fix

    default ResponseEntity<Void> testPost(
        @Parameter(name = "file", description = "", required = true) @RequestPart(value = "file", required = true) MultipartFile file,
        @Parameter(name = "content", description = "", required = true) @Valid @RequestPart(value = "content", required = true) TestObjectPart content
    ) {
        return new ResponseEntity<>(HttpStatus.NOT_IMPLEMENTED);

    }

Steps to reproduce

check out the reproduction repo: https://github.com/cc-ju/openapi-genrator-spring-error-reproduction

run ./gradlew check --info

To switch to the older generator just change:

• https://github.com/cc-ju/openapi-genrator-spring-error-reproduction/blob/main/build.gradle#L3-L4
• https://github.com/cc-ju/openapi-genrator-spring-error-reproduction/blob/main/build.gradle#L21-L24

Related issues/PRs

As mentioned above this regression is caused by #11449

Suggest a fix

Obviously #11449 resolved another problem. But broke this use case.
Currently I don't have a solution which does not break either this use case or the use case from the PR.

Start with the linked reproduction repository, its openapi.yaml, and the ./gradlew check --info command. Compare generated output with generator 5.x and 6.x, then trace the change from PR #11449. Done means multipart file and object parameters are converted correctly without regressing the use case addressed by that PR.