[BUG] content type multipart/formdata not set when generating a kotlin client

Created by: mthoolen

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

If i generate a kotlin client with the openapi generator, the multipart/form-data content type is not properly set

openapi-generator version

4.3

OpenAPI declaration file content or url

Command line used for generation

Steps to reproduce

1. generate a kotlin client with the supplied specification
2. check the value localVariableHeaders in the create method of the ExampleApi class. it should not contain a value.

Related issues/PRs

haven't found any related issues.

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