[BUG] [SPRING-CLOUD] UNKNOWN_BASE_TYPE when using multi-part content

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[x] Have you provided a full/minimal spec to reproduce the issue?
[x] Have you validated the input using an OpenAPI validator (example)?
[x] What's the version of OpenAPI Generator used?
[x] Have you search for related issues/PRs?
[x] What's the actual output vs expected output?
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Description

When I swap out a multip part in a request body object, the interface is generated incorrectly with UNKNOWN BASE TYPE

openapi-generator version

4.2.0

OpenAPI declaration file content or url

This yaml works

---
openapi: 3.0.2
info:
  title: Test
  version: 'v1'
  description: Test
paths:
  /foo:
    post:
      summary: Test
      operationId: createFoo
      requestBody:
        required: true
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                file:
                  type: array
                  items:
                    type: string
                    format: binary
      responses:
        '201':
          description: "Go"

    ResponseEntity<Void> createFoo(@ApiParam(value = "file detail") @RequestParam("file") MultipartFile file);

This yaml is not working

---
openapi: 3.0.2
info:
  title: Test
  version: 'v1'
  description: Test
paths:
  /foo:
    post:
      summary: Test
      operationId: createFoo
      requestBody:
        $ref: '#/components/requestBodies/MultiPartFile'
      responses:
        '201':
          description: "Go"
components:
  requestBodies:
    MultiPartFile:
      content:
        multipart/form-data:
          schema:
            type: object
            properties:
              file:
                type: array
                items:
                  type: string
                  format: binary
      description: Payload or user input is invalid. See error details in the payload for more information.

  ResponseEntity<Void> createFoo(@ApiParam(value = "Payload or user input is invalid. See error details in the payload for more information."  )  @Valid @RequestBody UNKNOWN_BASE_TYPE UNKNOWN_BASE_TYPE);

Command line used for generation

 <generatorName>spring</generatorName>
  <library>spring-cloud</library>
   <java8>true</java8>

Same here:

[main] WARN  o.o.codegen.DefaultCodegen - codegenModel is null. Default to UNKNOWN_BASE_TYPE

I had this in my components: requestBodies:

components:
  requestBodies:
      hashFileBody:
          content:
              multipart/form-data:
                  schema:
                     type: object
                  ...

and used it in some post:requestBody as a $ref. It didn't work there. After moving it up to the post:requestBody: (meaning no $ref) it worked again. Then, I tried to move only the schema part down to components:schemas which worked without any problems.

At the same time, this one here works in components:requestBodies:

components:
  requestBodies:
      fileUpload:
        content:
            application/octet-stream:
                schema:
                    type: string
                    format: binary

To me it looks like the multipart/form-data does not work as a $ref to requestBodies.

Version: 4.3.0-SNAPSHOT Language: dart

I think this is not a "spring-cloud" only issue (as tagged in the title). it affects other languages too.

OpenAPITools / openapi-generator