如何在Flutter MultipartRequest中获取字段value []？

Question

String token = await SharedPrefUtils.readPrefStr("token");
var uri = Uri.parse(Constants.BASEURL + Constants.KEYEORD_CREATE_TENANACY);

 Map<String, String> headers = {
"Accept": "application/json",
"Authorization": token
};

var request = new http.MultipartRequest("POST", uri);

request.headers.addAll(headers);

List<String> tags = ['1471','1765'];
String jsonTags = jsonEncode(tags);
print(jsonTags);

request.fields['property_usage'] = property_use;

request.fields['agent_id'] = agent_id;

request.fields['property_id'] = property_id;

request.fields['unit_id'] = jsonTags;  // here not getting value 

var response = await request.send();

如何使用MultipartRequest中的数组值将文件发送到服务器。上面我尝试添加json值，但这对我没有帮助