[BUG] [Kotlin] Bug generating array of files multipart

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

When generating API code using kotlin, with the use of the jvm-retrofit2 library. An issue occurs when generating code for an array of files. What it generates:

    @Multipart
    @POST("api/Example/Send")
    fun messagesSend(@Part attachments: MultipartBody.Part? = null): Completable

What is expected and works with the actual backend implementation

    @Multipart
    @POST("api/Example/Send")
    fun messagesSend(@Part attachments: Array<MultipartBody.Part?>? = null): Completable

It doesn't generate the attachments as a list, this way only 1 attachment can be uploaded. Api specification also includes that it should be an array.

openapi-generator version

6.2.1

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Example.API
  version: v1
paths:
  /api/Example/Send:
    post:
      tags:
        - Example
      operationId: SendExample
      requestBody:
        content:
          multipart/form-data:
            schema:
              required:
                - Content
                - Title
              type: object
              properties:
                attachments:
                  type: array
                  items:
                    type: string
                    format: binary
                  nullable: true
            encoding:
              attachments:
                style: form
      responses:
        '200':
          description: Success

Generation Details

Complete config

openApiGenerate {
    generatorName = "kotlin"
    library = "jvm-retrofit2"
    configOptions = [
            useRxJava3: "true",
            serializationLibrary: "gson",
            artifactId: "api-client",
            serializableModel: "true"
    ]
}

Steps to reproduce

I created an example repository for this issue. After running the gradle task: 'openApiGenerate' it will generate the OpenApi code, where you will see that it generates a single item as previously stated.
https://github.com/StijnAppec/OpenApiMultipartIssue

Suggest a fix

Solution would be that it generates the signature so that it includes a list of multipart part's. Instead of a single one.

Reproduce the issue with the linked example repository and the Gradle openApiGenerate task using the Kotlin jvm-retrofit2 configuration. Compare the generated multipart method with the OpenAPI declaration's array of binary attachments; done means the generated client signature preserves multiple multipart parts.