[BUG] [Spring] Generated API interface treats multipart arrays as a single file

Created by: galleog

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

Generated API interface for multipart arrays contains a single MultipartFile instead of a list. Besides any MultipartFile argument should have @RequestPart annotation instead of @RequestParam.

openapi-generator version

Tested with 4.0.0 version of https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator-maven-plugin

OpenAPI declaration file content or url

openapi: '3.0.0'
info:
  version: 1.0.0
  title: MultipartFile test
paths:
  /multipart-array:
    post:
      tags:
        - multipart
      description: MultipartFile array test
      operationId: multipartArray
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                files:
                  type: array
                  items:
                    type: string
                    format: binary
      responses:
        '204':
          description: Successful operation
  /multipart-single:
    post:
      tags:
        - multipart
      description: Single MultipartFile test
      operationId: multipartSingle
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                file:
                  type: string
                  format: binary
      responses:
        '204':
          description: Successful operation

Command line used for generation

Maven plugin was used:

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>${openapi-generator-maven-plugin.version}</version>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <inputSpec>${basedir}/src/main/resources/multipart.yaml</inputSpec>
                <generatorName>spring</generatorName>
                <modelPackage>org.openapi.test.model</modelPackage>
                <apiPackage>org.openapi.test.api</apiPackage>
                <library>spring-cloud</library>
                <generateSupportingFiles>false</generateSupportingFiles>
                <generateApiTests>false</generateApiTests>
                <configOptions>
                    <interfaceOnly>true</interfaceOnly>
                    <dateLibrary>java8</dateLibrary>
                    <java8>true</java8>
                    <booleanGetterPrefix>is</booleanGetterPrefix>
                    <useBeanValidation>true</useBeanValidation>
                    <useTags>true</useTags>
                </configOptions>
            </configuration>
        </execution>
    </executions>
</plugin>

Steps to reproduce

Generate MultipartApi interface using the plugin configuration listed above. Both multipartArray and multipartSingle methods contains a single MultipartFile parameter with @RequestParam annotation.

Related issues/PRs

Suggest a fix

Looks like the mustache template formParams.mustache ignores arrays.