[BUG] [cpp-qt5] Array of files generates a solution not compilable

[Mariolismo]

Description

The auto-generated code doesn't compile (after running CMake to generate the solution).
The problem seems to be related to this part of the YAML:

  file:                  
  type: array
  items:
    type: string
    format: binary

This is an array of files, inside the cpp-qt5 generated code is declared as a ::OpenAPI::OptionalParam<QList<OAIHttpFileElement>> &file but than is accessed like if it was a single element: file.value().local_filename, causing the compilation to fail.

openapi-generator version

5.1.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Processing Service
  version: 1.0.0
servers:
  - url: 'http://localhost:8080'
    description: Processing Service (Aresys GRAPeS Library)
paths:
  '/api/companies/{companyId}/mask':
    post:
      tags:
        - maskEndpoint
      operationId: uploadMask
      parameters:
        - name: companyId
          in: path
          required: true
          schema:
            type: integer
            format: int64
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                mask:
                  $ref: '#/components/schemas/MaskInfo'
                file:
                  type: array
                  items:
                    type: string
                    format: binary
        required: true
      responses:
        '200':
          description: OK
          content:
            '*/*':
              schema:
                type: string
  
components:
  schemas:
    MaskInfo:
      required:
        - maskId
        - timestamp
      type: object
      properties:
        maskId:
          type: array
          items:
            type: string
        timestamp:
          type: string
          format: date-time

Command line used for generation

java -DskipFormModel=false -jar openapi-generator-cli-5.1.0.jar generate -i openapi.yaml -g cpp-qt5-client -o qt5

Steps to reproduce

1. Execute the command in the folder containing the YAML file and the openapi-generator-cli-5.1.0.jar
2. Execute cmake client
3. Open the vs solution "client.sln" and rebuild

Related issues/PRs

Seems to be similar to: #3271

Suggest a fix/enhancement

The problem seems to happen only with cpp-qt5, since using SPRING the generators handles correcly the resulting list