[BUG][KOTLIN-SPRING] Multipart file upload generate wrong type

Created by: notizklotz

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?
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Description

The kotlin-spring generator generates multipart file upload attributes as org.springframework.core.io.Resource which actually cannot be handled by Spring Boot. org.springframework.web.multipart.MultipartFile should be generated instead: Uploading files guide

The spring generator handles this case correctly by applying some special case handling in the Mustache templates.

openapi-generator version

openapi-generator-maven-plugin:5.0.0

OpenAPI declaration file content or url

Multipart file upload sample spec

Steps to reproduce

Generate server code using the kotlin-spring generator
Generate server code using the spring generator
Note that the spring generator generates the file paramter as List<MultipartFile> while the kotlin-spring generator generates List<Resource>

Related issues/PRs

Suggest a fix

Apply the same special case handling in the templates like the spring generator