[kotlin-spring] Handle arrays of files correctly using MultipartFile (#20108)

* [kotlin-spring] Fix no List being used for an array of files using multipart/form-data

* [kotlin-spring] Use Spring's MultipartFile class for incoming Files instead of Spring's Resource class.

* [kotlin-spring] Add test to ensure that return type for files is `org.springframework.core.io.Resource`

* [kotlin-spring] Ensure Array is used for lists of files in generated Api class

* Update samples

* [kotlin-spring] Move conditional usage of MultipartFile to optionalDataType.mustache. Update samples

* update samples

---------

Co-authored-by: William Cheng <wing328hk@gmail.com>

Author: ffffionn

modules/openapi-generator/src/main/resources/kotlin-spring/apiDelegate.mustache

@@ -6,7 +6,6 @@ import org.springframework.http.HttpStatus
 import org.springframework.http.MediaType
 import org.springframework.http.ResponseEntity
 import org.springframework.web.context.request.NativeWebRequest
-import org.springframework.core.io.Resource
 {{#appendRequestToHandler}}
 import org.springframework.http.server.reactive.ServerHttpRequest
 {{/appendRequestToHandler}}
@@ -33,7 +32,7 @@ interface {{classname}}Delegate {
     /**
      * @see {{classname}}#{{operationId}}
      */
-    {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{{paramName}}}: {{^isFile}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}{{#isBodyParam}}Flow<{{{baseType}}}>{{/isBodyParam}}{{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{/isArray}}{{/reactive}}{{/isFile}}{{#isFile}}Resource?{{/isFile}}{{^-last}},
+    {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{{paramName}}}: {{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}{{#isBodyParam}}Flow<{{{baseType}}}>{{/isBodyParam}}{{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{/isArray}}{{/reactive}}{{^-last}},
         {{/-last}}{{/allParams}}): {{#responseWrapper}}{{.}}<{{/responseWrapper}}ResponseEntity<{{>returnTypes}}>{{#responseWrapper}}>{{/responseWrapper}}{{^skipDefaultDelegateInterface}} {
         {{>methodBody}}
     }{{/skipDefaultDelegateInterface}}

modules/openapi-generator/src/main/resources/kotlin-spring/optionalDataType.mustache

@@ -1 +1 @@
-{{#required}}{{{dataType}}}{{/required}}{{^required}}{{#defaultValue}}{{{dataType}}}{{/defaultValue}}{{^defaultValue}}{{{dataType}}}?{{/defaultValue}}{{/required}}
+{{^isFile}}{{{dataType}}}{{^required}}{{^defaultValue}}?{{/defaultValue}}{{/required}}{{/isFile}}{{#isFile}}{{#isArray}}Array<{{/isArray}}org.springframework.web.multipart.MultipartFile{{#isArray}}>{{/isArray}}{{^isArray}}{{^required}}?{{/required}}{{/isArray}}{{/isFile}}