PHP support for multipart/form-data POST request

Description

trying to generate a PHP client library, using multipart/form-data parameters for POST request.
i see in the PHP api.mustache that $multipart is initially set to false and only if a file parameter has been received it it's value is changed to true:

        // form params
        if (${{paramName}} !== null) {
            {{#isFile}}
            $multipart = true;
            $formParams['{{baseName}}'] = \GuzzleHttp\Psr7\try_fopen(ObjectSerializer::toFormValue(${{paramName}}), 'rb');
           {{/isFile}}
            {{^isFile}}
            $formParams['{{baseName}}'] = ObjectSerializer::toFormValue(${{paramName}});
            {{/isFile}}
        }

Later in the same file we see that only if $multipart is set to true a multipart request is created:

        if ($multipart) {
            $headers = $this->headerSelector->selectHeadersForMultipart(
                [{{#produces}}'{{{mediaType}}}'{{#hasMore}}, {{/hasMore}}{{/produces}}]
            );
        } else {
            $headers = $this->headerSelector->selectHeaders(
                [{{#produces}}'{{{mediaType}}}'{{#hasMore}}, {{/hasMore}}{{/produces}}],
                [{{#consumes}}'{{{mediaType}}}'{{#hasMore}}, {{/hasMore}}{{/consumes}}]
            );
        }

this is my swagger json file:

{
  "swagger": "2.0",
  "info": {
    "version": "v1",
    "title": "",
    "contact": {
      "name": ""
    },
    "license": {
      "name": "",
      "url": "http://licenseUrl"
    }
  },
  "host": "localhost:9000",
  "basePath": "/",
  "tags": [
    {
      "name": "Mail Controller"
    },
    {
      "name": "Health Check Controller"
    }
  ],
  "paths": {
    "/mail-service/submit": {
      "post": {
        "tags": [
          "Mail Controller"
        ],
        "summary": "Send Mail",
        "description": "",
        "operationId": "submit",
        "consumes": [
          "multipart/form-data"
        ],
        "produces": [
          "application/json"
        ],
        "parameters": [
          {
            "name": "from",
            "in": "formData",
            "required": true,
            "type": "string"
          },
          {
            "name": "Content-Type",
            "value" : "multipart/form-data",
            "in": "header",
            "type": "string"
          },
          {
            "name": "subject",
            "in": "formData",
            "required": true,
            "type": "string"
          },
          {
            "name": "body",
            "in": "formData",
            "required": true,
            "type": "string"
          },
          {
            "name": "to",
            "in": "formData",
            "required": true,
            "type": "string"
          },
          {
            "name": "cc",
            "in": "formData",
            "required": false,
            "type": "string"
          },
          {
            "name": "bcc",
            "in": "formData",
            "required": false,
            "type": "string"
          },
          {
            "name": "replyTo",
            "in": "formData",
            "required": false,
            "type": "string"
          },
          {
            "name": "file",
            "in": "formData",
            "required": false,
            "type": "file"
          }
        ],
        "responses": {
          "200": {
            "description": "success",
            "schema": {
              "$ref": "#/definitions/EmailJsonSuccessResponse"
            }
          },
          "400": {
            "description": "validation error",
            "schema": {
              "$ref": "#/definitions/JsonErrorResponse"
            }
          },
          "500": {
            "description": "server error",
            "schema": {
              "$ref": "#/definitions/JsonErrorResponse"
            }
          }
        }
      }
    }
  },
  "definitions": {
    "JsonErrorResponse": {
      "type": "object",
      "properties": {
        "code": {
          "type": "integer",
          "format": "int32"
        },
        "message": {
          "type": "string"
        },
        "emailId": {
          "type": "string"
        }
      }
    },
    "EmailJsonSuccessResponse": {
      "type": "object",
      "properties": {
        "code": {
          "type": "integer",
          "format": "int32"
        },
        "message": {
          "type": "string"
        },
        "emailId": {
          "type": "string"
        }
      }
    },
    "JsonResponse": {
      "type": "object",
      "properties": {
        "code": {
          "type": "integer",
          "format": "int32"
        },
        "message": {
          "type": "string"
        },
        "emailId": {
          "type": "string"
        }
      }
    }
  }
}

therefore, when sending a POST request with form parameters without a file parameter (not mandatory parameter), the generated client library does not send the request as a multipart/form-data parameters request.

Swagger-codegen version

version 2.3.1

Swagger declaration file content or url

Command line used for generation

java -DdebugOperations -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate
-i mail.json
-l php
-o ~/workspace/swagger-codegen/mail-service-php

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

what might fix it is to set the multipart to true not only if a file was received as a parameter, but to check the content-type header parameter instead.

Start in the PHP api.mustache template, focusing on how form parameters set $multipart and how the request headers are selected. Reproduce the provided Swagger definition with the PHP generator, then verify that a multipart/form-data request remains multipart when no file parameter is supplied.