[BUG] PHP client is not sending multipart request unless a file is part of the schema

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

Description

I noticed that even if the OpenAPI3 definition says it consumes multipart/form-data the generated client is not sending multipart request. It only sends multipart requests if a property of the type string and format binary is defined and used in the request (definition only is not changing anything). In my point of view the client should send what the server api has defined to be consumed.

openapi-generator version

4.2.3, 4.3.0 and 5.0.0

OpenAPI declaration file content or url

https://gist.github.com/reeperbahnause/07fc91a8c276e7479c270b8353eb45d9#file-openapi-generator-php-client-multipart-json

Command line used for generation

java -jar openapi-generator-cli-4.2.3.jar generate -i apitestminimal.json -g php -o apitestdir

Steps to reproduce

Generate the php client api send a request and check the request on the server side. It wont be a multipart/form-data request.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/3750

Suggest a fix

If the parameter is not a file, it might still be a multipart-form-data message, but it's not set to true?
I think, this is the spot to look at:
https://github.com/OpenAPITools/openapi-generator/blob/3588990a4f9f7e7d3e3ae8e2dda59206f57bf676/modules/openapi-generator/src/main/resources/php/api.mustache#L533-L541

Start with the PHP template at modules/openapi-generator/src/main/resources/php/api.mustache around lines 533-541, then reproduce the request using the command and OpenAPI declaration linked in the issue. Review related PR #3750 and trace how multipart handling is selected when the schema has no binary property. Done means the generated PHP client sends multipart/form-data for the described multipart endpoint.