dio_form_data_bytes.dart

import 'package:dio/dio.dart';  

Future<dynamic> dioPostFormData(
    String url, {
    Map<String, String> headers,
    Map<String, dynamic> body,
    String method = 'POST',
    Map<String, Uint8List> files,
  }) async {
    
    FormData formData;
  
    // this will be for regular content other than file
    formData = FormData.fromMap(body);
  
    
    if (files != null) {
      for (MapEntry<String, Uint8List> entry in files.entries) {
        MapEntry<String, MultipartFile> mapEntry = MapEntry(
          entry.key,
          MultipartFile.fromBytes(
            uint8list,
          ),
        );
        formData.files.add(mapEntry);
      }
    } 
  
    Dio dio = new Dio(BaseOptions(
      headers: headers,
      receiveTimeout: 60 * 1000,
      sendTimeout: 60 * 1000,
    ));
    var response;
    
 
    response = await dio
        .post(
          url,
          data: formData,
        );
    return response.data;
  }

dio_form_data_file.dart

import 'package:dio/dio.dart';  

Future<dynamic> dioPostFormData(
    String url, {
    Map<String, String> headers,
    Map<String, dynamic> body,
    String method = 'POST',
    Map<String, File> files,
  }) async {
    
    FormData formData;
  
    // this will be for regular content other than file
  // if you dont have any extra data simply pass an empty map like <String,dynamic>{} for body field. 
    formData = FormData.fromMap(body);
  
    
    if (files != null) {
      for (MapEntry<String, File> entry in files.entries) {
        MapEntry<String, MultipartFile> mapEntry = MapEntry(
          entry.key,
          // in this example I used a file object, to use the Uint8list you can use MultipartFile.fromBytes method
          // i have modified it below in another example so you can use that and see
          await MultipartFile.fromFile(
            entry.value.path,
            filename: basename(entry.value.path),
          ),
        );
        formData.files.add(mapEntry);
      }
    } 
  
    Dio dio = new Dio(BaseOptions(
      headers: headers,
      receiveTimeout: 60 * 1000,
      sendTimeout: 60 * 1000,
    ));
    var response;
    
 
    response = await dio
        .post(
          url,
          data: formData,
        );
    return response.data;
  }