[BUG] `ApiClient.kt.mustache` calls `.toString()` method on a `ByteArray` when building multipart request

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

In the ApiClient.kt.mustache file during the creation of the request body the requestBody(content: T, mediaType: String?) method correctly builds a multipart/form request when part.body is File, but for all other data types the part is added to the request by calling the .toString() method on the part body:

addPart(
    Headers.of(partHeaders),
    RequestBody.create(null, parameterToString(part.body))
)

This results in the .toString() method being called on a ByteArray - which returns a string containing the class name, '@' symbol, and the hash code of the array object ("[B@7c67645b"), instead of the contents of the byte array.

openapi-generator version

v7.1.0

OpenAPI declaration file content or url

Generation Details

openapi: 3.0.1
info:
  contact:
    email: na@na.com
  description: OpenAPI definitions
  license:
    name: Apache 2.0
    url: http://www.apache.org/licenses/LICENSE-2.0.html
  termsOfService: https://terms.io/
  title: API
  version: 16.0.0
security:
- basicAuth: []
- bearerAuth: []
paths:
  /path/to/resource:
    post:
      operationId: createReceipt
      parameters:
      - in: path
        name: testId
        required: true
        schema:
          type: string
          format: uuid
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                file:
                  type: object

Steps to reproduce

From the given api spec, generate a Kotlin Api client and POST a ByteArray in the request.

Related issues/PRs

Suggest a fix

Start in modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/infrastructure/ApiClient.kt.mustache, especially requestBody(content: T, mediaType: String?) and the multipart addPart call around line 156. Reproduce the issue by generating a Kotlin client from the supplied multipart OpenAPI definition and posting a ByteArray; done means the request contains the byte array contents rather than its object representation.