[BUG] Multiple file upload + metadata fails as multipart, works as json+base64

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue? --> https://github.com/sebpiller/openapi-bug
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists? --> No, only with 6.0.1 - master does not build at the moment (module java.base does not "opens java.util" to unnamed module xxx) !
• Have you searched for related issues/PRs?
• What's the actual output vs expected output? --> Generation error vs generation done: [WARNING] Could not compute datatypeWithEnum from UploadFileModel, null

Description

I'm trying to use openapi + spring boot to declare an endpoint where I can upload multiple files, each of them having metadatas (for example, an "urgent" flag that can be set for each file). I tried to use a multipart/form-data endpoint with a binary property but the generation of the beans&api is failing with a warning: [WARNING] Could not compute datatypeWithEnum from UploadFileModel, null

If I change the code to use a non-multipart request, eg. application/json+base64encoding for the upload file, then the issue disappear.

The code is available as a minimal reproducible example here: https://github.com/sebpiller/openapi-bug (toggle comment at demo.yaml, lines 11+12, 44+45 to have a working implem)

openapi-generator version

6.0.1, but fails similarly with all version I've tested

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Demo
  version: '1.0'
paths:
  '/demo/ok':
    post:
      operationId: ok
      requestBody:
        content:
          multipart/form-data:
            schema:
              $ref: '#/components/schemas/UploadFilesRequest'
      responses:
        '200':
          description: OK
          headers: { }
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/UploadResponse'
components:
  schemas:
    UploadFilesRequest:
      title: Upload several files
      required:
        - files
      properties:
        files:
          type: array
          items:
            $ref: '#/components/schemas/UploadFile'

    UploadFile:
      title: A file upload with some metadata
      properties:
        urgent:
          type: boolean
        filename:
          type: string
        data:
          type: string
          format: binary

    UploadResponse:
      title: A stub response
      properties:
        accepted:
          type: boolean

Generation Details

Maven generator 6.0.1, see linked project's pom

Steps to reproduce

Run clean install on the demo project. It fails with a warning "[WARNING] Could not compute datatypeWithEnum from UploadFileModel, null"

Switching to application/json + base64 encoding of uploads works, but is suboptimal

Start with the linked minimal reproduction, especially demo.yaml and its Maven pom, then run clean install to reproduce the UploadFileModel warning. Trace how the generator handles multipart/form-data schemas containing arrays of binary-bearing objects. Done means the sample generates successfully without the warning while preserving the multipart upload and metadata structure.