Why http request returned empty

I write the code to send a binary data of an recorded speech audio "tmphpzsjbv7.flac" file to Google speech website to get the text of the speech:

 URL = 'http://www.google.com/speech-api/v2/recognize?client=chromium&lang=zh-CN&key=AIzaSyBOti4mM-6x9WDnZIjIeyEU21OpBXqWBgw';
                filename = 'D:\QMDownload\1\tmphpzsjbv7.flac';
                fid = fopen(filename,'r+');
                data1 = uint8(fread(fid))';
                fclose(fid);
            header = matlab.net.http.field.ContentTypeField('audio/x-flac; rate=16000');
            data = char(data1);
            req = matlab.net.http.RequestMessage('POST',header,data);
            uri = matlab.net.URI(URL);
            resp = req.send(uri); 
            resp.Body.Data.result % always is []
            

When I run the equivalent code in py module of matlab using the requests lib imported from py, it indeed returns an resp object, and the resp.content field (a very long string) has the recognized speech inside, and I just need to dig it out using regexp.

[raym]

below is the returned resp:

resp.char

ans =

    'HTTP/1.1 200 OK
     Content-Type: application/json; charset=utf-8
     Content-Disposition: attachment
     Cache-Control: no-transform
     X-Content-Type-Options: nosniff
     Pragma: no-cache
     Date: Mon, 26 Nov 2018 14:04:20 GMT
     Server: S3 v1.0
     X-XSS-Protection: 1; mode=block
     X-Frame-Options: SAMEORIGIN
     Accept-Ranges: none
     Vary: Accept-Encoding
     Connection: close
     {"result":[]}