diff --git a/clients/java/src/test/java/com/phrase/client/api/UploadsApiTest.java b/clients/java/src/test/java/com/phrase/client/api/UploadsApiTest.java
index 7b3e6832..a19a8408 100644
--- a/clients/java/src/test/java/com/phrase/client/api/UploadsApiTest.java
+++ b/clients/java/src/test/java/com/phrase/client/api/UploadsApiTest.java
@@ -37,6 +37,8 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 /**
  * API tests for UploadsApi
@@ -104,18 +106,29 @@ public void uploadCreateTest() throws ApiException, IOException, InterruptedExce
         Boolean skipUnverification = null;
         String fileEncoding = null;
         Object localeMapping = null;
-        Object formatOptions = null;
         Boolean autotranslate = null;
         Boolean markReviewed = null;
         Boolean tagOnlyAffectedKeys = null;
-        Upload response = api.uploadCreate(projectId, file, fileFormat, localeId, xPhraseAppOTP, branch, tags, updateTranslations, updateTranslationKeys, updateTranslationsOnSourceMatch, updateDescriptions, convertEmoji, skipUploadTags, skipUnverification, fileEncoding, localeMapping, formatOptions, autotranslate, markReviewed, tagOnlyAffectedKeys);
+
+        Map<String, String> nestedFormatOptionsMap = new HashMap<>();
+        nestedFormatOptionsMap.put("nested_option", "sub_option");
+
+        Map<String, Object> formatOptionsMap = new HashMap<>();
+        formatOptionsMap.put("omit_separator_space", "true");
+        formatOptionsMap.put("fallback_language", "en");
+        formatOptionsMap.put("more_options", nestedFormatOptionsMap);
+
+        Upload response = api.uploadCreate(projectId, file, fileFormat, localeId, xPhraseAppOTP, branch, tags, updateTranslations, updateTranslationKeys, updateTranslationsOnSourceMatch, updateDescriptions, convertEmoji, skipUploadTags, skipUnverification, fileEncoding, localeMapping, formatOptionsMap, autotranslate, markReviewed, tagOnlyAffectedKeys);
 
         Assert.assertEquals("valid id returned", "id_example", response.getId());
         Assert.assertEquals("valid creation date returned", OffsetDateTime.parse("2015-01-28T09:52:53Z"), response.getCreatedAt());
 
         RecordedRequest recordedRequest = mockBackend.takeRequest();
         Assert.assertEquals("Request path", "//projects/projectId_example/uploads", recordedRequest.getPath());
-        Assert.assertTrue("Request payload", recordedRequest.getBody().readUtf8().contains("Content-Disposition: form-data; name=\"file\""));
+        String requestBody = recordedRequest.getBody().readUtf8();
+        Assert.assertTrue("payload contains filename", requestBody.contains("Content-Disposition: form-data; name=\"file\""));
+        Assert.assertTrue("payload contains fileFormat", Pattern.compile("Content-Disposition: form-data; name=\"file_format\"\\s+Content-Length: 11\\s+simple_json", Pattern.MULTILINE).matcher(requestBody).find());
+        Assert.assertTrue("payload contains nested format options", Pattern.compile("Content-Disposition: form-data; name=\"format_options\\[more_options\\]\\[nested_option\\]\"\\s+Content-Length: 10\\s+sub_option", Pattern.MULTILINE).matcher(requestBody).find());
     }
 
     /**
diff --git a/openapi-generator/templates/java/libraries/okhttp-gson/ApiClient.mustache b/openapi-generator/templates/java/libraries/okhttp-gson/ApiClient.mustache
index 44f1275d..23af378c 100644
--- a/openapi-generator/templates/java/libraries/okhttp-gson/ApiClient.mustache
+++ b/openapi-generator/templates/java/libraries/okhttp-gson/ApiClient.mustache
@@ -667,14 +667,11 @@ public class ApiClient {
                 {{! TODO check why nested params ordering is different }}
                 params.addAll(parsedMappedParams(key, value, new ArrayList<Pair>()));
             }
-
         }
-        
 
         return params;
     }
 
-
     /**
      * Formats the specified query parameter to a list containing a single {@code Pair} object.
      *
@@ -1348,8 +1345,12 @@ public class ApiClient {
                 MediaType mediaType = MediaType.parse(guessContentTypeFromFile(file));
                 mpBuilder.addPart(partHeaders, RequestBody.create(mediaType, file));
             } else {
-                Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"");
-                mpBuilder.addPart(partHeaders, RequestBody.create(null, parameterToString(param.getValue())));
+                // flatten the nested structures first
+                List<Pair> flatParams = parsedMappedParams(param.getKey(), param.getValue(), new ArrayList<Pair>());
+                for (Pair pair : flatParams) {
+                    Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + pair.getName() + "\"");
+                    mpBuilder.addPart(partHeaders, RequestBody.create(null, parameterToString(pair.getValue())));
+                }
             }
         }
         return mpBuilder.build();
@@ -1473,11 +1474,11 @@ public class ApiClient {
 
             for(Map.Entry<String, Object> entry : mappedValue.entrySet()){
                 String nestedKey = key + "[" + entry.getKey() + "]";
-                parsedMappedParams(nestedKey, entry.getValue(), params); 
+                parsedMappedParams(nestedKey, entry.getValue(), params);
             }
         }
         else{
-            params.add(new Pair(key, parameterToString(value)));    
+            params.add(new Pair(key, parameterToString(value)));
         }
 
         return params;