你可以通过下面的代码试试：

try {
          FormData formData;
          List<dynamic>? documents = [];
          for (int i = 0; i < file.length; i++) {
            String? fileName = file[i]?.path.split('/').last;
            documents.add(await MultipartFile.fromFile(file[i]?.path ?? '', filename: fileName));
          }
          formData = FormData.fromMap({
            'files[]': documents,
          });
          final response = await _dio.post("/medias", data: formData);
          return response.data;
     }

在您的上传方法中：

Map<String, dynamic> postData = Map<String, dynamic>();

postData["attachment[]"] = [];

for (File item in file) {
  postData["attachment[]"].add(await MultipartFile.fromFile(file.path, filename: basename(file.path)));
}

final formData = FormData.fromMap(postData);