[BUG][SPRING] Reactive generates wrong datatype for MultipartFile

Description

When I use the optional configuration to use reactive the codegen generates the wrong datatype for a multipartfile.

openapi-generator version

4.2.0

OpenAPI declaration file content or url

Actual

    default Mono<ResponseEntity<AttachmentModel>> createAttachment(@ApiParam(value = "") @RequestParam(value="fileName", required=false)  String fileName,@ApiParam(value = "file detail") @Valid @RequestPart("file") MultipartFile file, ServerWebExchange exchange) throws Exception {

Expected

    default Mono<ResponseEntity<AttachmentModel>> createAttachment(@ApiParam(value = "") @RequestParam(value="fileName", required=false)  String fileName,@ApiParam(value = "file detail") @Valid @RequestPart("file") Mono<FilePart> file, ServerWebExchange exchange) throws Exception {

Command line used for generation

<generatorName>spring</generatorName>
<modelNameSuffix>Model</modelNameSuffix>
<configOptions>
  <sourceFolder>src/gen/java/main</sourceFolder>
  <unhandledException>true</unhandledException>
  <interfaceOnly>true</interfaceOnly>
  <java8>true</java8>
  <dateLibrary>java8</dateLibrary>
  <booleanGetterPrefix>is</booleanGetterPrefix>
  <reactive>true</reactive>
</configOptions>

Suggest a fix

In the mustache file check for reactive and use the right datatype.

Start in the Spring generator templates indicated by the suggested fix, using the reactive=true configuration and the MultipartFile example as the reproduction. Compare the generated parameter type with the expected Mono output, then verify that reactive multipart generation produces the expected signature.