diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/resteasy/ApiClient.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/resteasy/ApiClient.mustache
index 725e01185296..c15c0e9e0c61 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/resteasy/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/resteasy/ApiClient.mustache
@@ -9,6 +9,7 @@ import java.io.UnsupportedEncodingException;
 import java.net.URLEncoder;
 import java.nio.file.Files;
 import java.nio.file.Paths;
+import java.nio.file.StandardCopyOption;
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
 import java.util.ArrayList;
@@ -495,7 +496,14 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
       MultipartFormDataOutput multipart = new MultipartFormDataOutput();
       //MultiPart multiPart = new MultiPart();
       for (Entry<String, Object> param: formParams.entrySet()) {
-        if (param.getValue() instanceof File) {
+        if( param.getValue() instanceof List && !( ( List ) param.getValue() ).isEmpty()
+                  && ( ( List ) param.getValue() ).get( 0 ) instanceof File ) {
+            @SuppressWarnings( "unchecked" )
+            List<File> files = ( List<File> ) param.getValue();
+            for( File file : files ) {
+              multipart.addFormData(param.getKey(),new FileInputStream(file),MediaType.APPLICATION_OCTET_STREAM_TYPE, file.getName());
+            }
+        } else if (param.getValue() instanceof File) {
           File file = (File) param.getValue();
           try {
             multipart.addFormData(param.getKey(),new FileInputStream(file),MediaType.APPLICATION_OCTET_STREAM_TYPE, file.getName());
@@ -534,10 +542,10 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
       return null;
     }
 
-    if ("byte[]".equals(returnType.toString())) {
+    if (returnType.getRawType().equals(byte[].class)) {
       // Handle binary response (byte array).
       return (T) response.readEntity(byte[].class);
-    } else if (returnType.equals(File.class)) {
+    } else if (returnType.getRawType().equals(File.class)) {
       // Handle file downloading.
       @SuppressWarnings("unchecked")
       T file = (T) downloadFileFromResponse(response);
@@ -563,7 +571,7 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
   public File downloadFileFromResponse(Response response) throws ApiException {
     try {
       File file = prepareDownloadFile(response);
-      Files.copy(response.readEntity(InputStream.class), file.toPath());
+      Files.copy(response.readEntity(InputStream.class), file.toPath(), StandardCopyOption.REPLACE_EXISTING);
       return file;
     } catch (IOException e) {
       throw new ApiException(e);