fix: kotlin multiplatform form-data file upload

Author: kalinjul

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java

@@ -1009,7 +1009,11 @@ public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<Mo
                 if ((JVM_KTOR.equals(getLibrary()) || MULTIPLATFORM.equals(getLibrary())) && operation.allParams != null) {
                     for (CodegenParameter param : operation.allParams) {
                         if (param.dataFormat != null && param.dataFormat.equals("binary")) {
-                            param.baseType = param.dataType = "io.ktor.client.request.forms.InputProvider";
+                            if (param.isContainer) {
+                                param.baseType = param.dataType = typeMapping.get(collectionType) + "<io.ktor.client.request.forms.FormPart<io.ktor.client.request.forms.InputProvider>>";
+                            } else {
+                                param.baseType = param.dataType = "io.ktor.client.request.forms.FormPart<io.ktor.client.request.forms.InputProvider>";
+                            }
                         }
                     }
                 }

modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/api.mustache

@@ -76,11 +76,11 @@ import kotlinx.serialization.encoding.*
                 {{#formParams}}
                 {{#isArray}}
                 {{{paramName}}}?.onEach {
-                    append("{{{baseName}}}[]", it)
+                    {{#isFile}}append(it){{/isFile}}{{^isFile}}append("{{{baseName}}}", it){{/isFile}}
                 }
                 {{/isArray}}
                 {{^isArray}}
-                {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}) }
+                {{{paramName}}}?.apply { {{#isFile}}append({{{baseName}}}){{/isFile}}{{^isFile}}append("{{{baseName}}}", {{^isEnumOrRef}}{{{paramName}}}{{/isEnumOrRef}}{{#isEnumOrRef}}{{{paramName}}}.value{{/isEnumOrRef}}){{/isFile}} }
                 {{/isArray}}
                 {{/formParams}}
             }