试试这个：

import 'dart:io';
import 'dart:convert';

File imageFile = new File(imageFilePath);
List<int> imageBytes = imageFile.readAsBytesSync();
String base64Image = BASE64.encode(imageBytes);

要发布没有base64编码的图像

final data = FormData.fromMap({
  'image_media': await MultipartFile.fromFile(
     imagePath,
      filename: filename),
});
try {
  final response =
      await this._dio.post('url', data: data);
  return response;}