我有下面的代码来发送一个录制的wav文件到wit.ai API。

Map<String, String> headers = {
        'Content-Type': 'audio/wav',
        'Authorization': "Bearer $witPublicAPIKey"
      };
      http.MultipartRequest request =
          http.MultipartRequest('POST', Uri.parse(BASE_URL + url));
      request.headers.clear();
      request.headers.addAll(headers);
      request.headers.update('content-type', (value) => 'audio/wav');
      request.headers.update('Content-Type', (value) => 'audio/wav');
      request.headers.update('Content-Type', (value) => 'audio/wav');
      request.files.add(await http.MultipartFile.fromPath('audio', audioUri,
          filename: '${audioUri.split('/').last}.wav',
          contentType: MediaType('audio', 'wav')));
      print("request header is ${request.headers}");
      print("request is ${request}");
      http.Response res = await http.Response.fromStream(await request.send());
      print("Response: ${res.body}");

      responseJson = _response(res);

我得到这个错误"error": "Unsupported content-type: 'multipart/form-data;boundary=dart-http-boundary-bpY_n6t9LVcEAVFf7qGYHXDU.H_s1alCjaGGk++6ZOcJ5M4iRFT'"作为响应。从docs看，HTTP覆盖了我的content-type: audio/wav头并将其替换为multipart/form-data。我想通过我的Flutter应用程序上传wav。你知道我该怎么做吗谢谢。