[BUG] Codegen isMultipart always false

Created by: ybelenko

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

Codegen isMultipart property is always false even when multipart/form-data in consumes array.

Related codegen part:

{
  "responseHeaders": [],
  "hasAuthMethods": true,
  "hasConsumes": true,
  "hasProduces": true,
  "hasParams": true,
  "hasOptionalParams": true,
  "hasRequiredParams": true,
  "returnTypeIsPrimitive": false,
  "returnSimpleType": true,
  "subresourceOperation": false,
  "isMapContainer": false,
  "isListContainer": false,
  "isMultipart": false,
  "hasMore": false,
  "isResponseBinary": false,
  "isResponseFile": false,
  "hasReference": true,
  "isRestfulIndex": false,
  "isRestfulShow": false,
  "isRestfulCreate": false,
  "isRestfulUpdate": false,
  "isRestfulDestroy": false,
  "isRestful": false,
  "isDeprecated": false,
  "isCallbackRequest": false,
  "path": "/fake/{petId}/uploadImageWithRequiredFile",
  "operationId": "uploadFileWithRequiredFile",
  "returnType": "\\OpenAPIServer\\Model\\ApiResponse",
  "httpMethod": "POST",
  "returnBaseType": "ApiResponse",
  "summary": "uploads an image (required)",
  "baseName": "Pet",
  "consumes": [
    {
      "mediaType": "multipart/form-data"
    }
  ]
}

openapi-generator version

4.1.1

OpenAPI declaration file content or url

petstore-with-fake-endpoints-models-for-testing.yaml

Command line used for generation

Build any SDK as usual with -DdebugOperations option.

$ java -jar ./modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -DdebugOperations -t modules/openapi-generator/src/main/resources/php-slim-server -i modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml -g php-slim -o samples/server/petstore/php-slim

Steps to reproduce

Generate any language library with -DdebugOperations option. Check operation uploadFileWithRequiredFile output.