iOS 利用http上传图片出错

你好，我根据后台返回的oss_access_key_id，url，policy，signature，做post请求，但是请求出错，实在不知道问题出在哪里

下面是代码片段：

• (void)postOSSImageKeybucketName:(NSString )oss_access_key_id url:(NSString )url policy:(NSString )policy Signature:(NSString )signature{

  if (_currentImg == nil || !_currentImg) {

  return;

  }
  //2. 图片名字
  NSString *fileName = [ShareMethod ret20bitString];
  fileName = [fileName stringByAppendingString:@".jpg"];
  //3. 图片二进制文件
  NSData *imagedata = UIImageJPEGRepresentation(_currentImg, 0.00001);
  //4. 发起网络请求
  MySessionManager *manager = [MySessionManager manager];
  NSMutableDictionary *dic = [[NSMutableDictionary alloc]init];
  [dic setValue:fileName forKey:@"key"];
  [dic setValue:oss_access_key_id forKey:@"OSSAccessKeyId"];
  [dic setValue:policy forKey:@"policy"];
  [dic setValue:signature forKey:@"Signature"];
  [dic setValue:imagedata forKey:@"file"];
  // [dic setValue:fileName forKey:@"MyFilename.jpg"];

// [dic setValue:@"200" forKey:@"success_action_status"];

[manager POST:url parameters:dic constructingBodyWithBlock:^(id<AFMultipartFormData>  _Nonnull formData) {
    [formData appendPartWithFileData:imagedata name:fileName fileName:fileName mimeType:@"image/jpeg"];

} success:^(NSURLSessionDataTask * _Nonnull task, id  _Nonnull responseObject) {
    NSLog(@"%@",responseObject);

} failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
    NSHTTPURLResponse *response = (NSHTTPURLResponse *)task.response;
    NSLog(@"++++%ld",(long)response.statusCode);
}];

}