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

Bug Report Checklist

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

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>

Start with the two OpenAPI YAML examples and the spring generator command using the spring-cloud library; compare generation for the inline multipart request body and the referenced request body. Trace why the referenced schema becomes UNKNOWN_BASE_TYPE, and verify that the generated method uses the expected MultipartFile parameter instead.