Fix multipart requests for additional-properties

Author: sermler

modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/api.mustache

@@ -140,7 +140,7 @@ public class {{classname}} extends BaseApi {
         {{/-last}}{{/cookieParams}}{{/hasCookieParams}}{{#hasFormParams}}
 
         {{#formParams}}if ({{paramName}} != null)
-            localVarFormParams.{{^collectionFormat}}add{{/collectionFormat}}{{#collectionFormat}}addAll{{/collectionFormat}}("{{baseName}}", {{#isFile}}{{^collectionFormat}}{{#useAbstractionForFiles}}{{paramName}}{{/useAbstractionForFiles}}{{^useAbstractionForFiles}}new FileSystemResource({{paramName}}){{/useAbstractionForFiles}}{{/collectionFormat}}{{/isFile}}{{#isFile}}{{#collectionFormat}}{{paramName}}.stream(){{^useAbstractionForFiles}}.map(FileSystemResource::new){{/useAbstractionForFiles}}.collect(Collectors.toList()){{/collectionFormat}}{{/isFile}}{{^isFile}}{{paramName}}{{/isFile}});{{^-last}}
+            localVarFormParams.{{^isArray}}add{{/isArray}}{{#isArray}}addAll{{/isArray}}("{{baseName}}", {{#isFile}}{{^collectionFormat}}{{#useAbstractionForFiles}}{{paramName}}{{/useAbstractionForFiles}}{{^useAbstractionForFiles}}new FileSystemResource({{paramName}}){{/useAbstractionForFiles}}{{/collectionFormat}}{{/isFile}}{{#isFile}}{{#collectionFormat}}{{paramName}}.stream(){{^useAbstractionForFiles}}.map(FileSystemResource::new){{/useAbstractionForFiles}}.collect(Collectors.toList()){{/collectionFormat}}{{/isFile}}{{^isFile}}{{paramName}}{{/isFile}});{{^-last}}
         {{/-last}}{{/formParams}}{{/hasFormParams}}
 
         final String[] localVarAccepts = { {{#hasProduces}}