Can't open Microsoft Speech Recognition API via Post with Delphi - delphi

Question

I want to send a Post request to Microsoft Speech Recognition API via HTTPS using Indy's TIdHTTP in Delphi.
On Microsofts Speech Recognition API Page: Microsoft Speech Recognition API Get started with speech recognition by using the REST API
they write you should send a HTTP POST Request like this:
POST https://speech.platform.bing.com/speech/recognition/interactive/cognitiveservices/v1?language=en-US&format=detailed HTTP/1.1
Accept: application/json;text/xml
Content-Type: audio/wav; codec=audio/pcm; samplerate=16000
Ocp-Apim-Subscription-Key: YOUR_SUBSCRIPTION_KEY
Host: speech.platform.bing.com
Transfer-Encoding: chunked
Expect: 100-continue
I try this with Delphi XE 10 Indy.
But I ever got Error 400 - Bad Request as Answer!
What do I false in the following code?
procedure TForm1.Button1Click(Sender: TObject);
var
Response, csrf, url: String;
PostStream: TIdMultipartFormDataStream;
HTTPClient: TIdHTTP;
SSL: TIdSSLIOHandlerSocketOpenSSL;
begin
url := 'https://speech.platform.bing.com/speech/recognition/interactive/cognitiveservices/v1?language=en-US&format=detailed HTTP/1.1';
HTTPClient := TIdHTTP.Create;
try
HTTPClient.Disconnect;
HTTPClient.AllowCookies := True;
SSL := TIdSSLIOHandlerSocketOpenSSL.Create(HTTPClient);
SSL.SSLOptions.SSLVersions := [sslvTLSv1, sslvTLSv1_1, sslvTLSv1_2];
HTTPClient.IOHandler := SSL;
HTTPClient.HandleRedirects := true;
HTTPClient.Request.Accept := 'application/json;text/xml';
HTTPClient.Request.Method := 'POST';
HTTPClient.Request.ContentType := 'audio/wav; codec=audio/pcm; samplerate=16000';
//-----------------------------------------------------------------------
PostStream := TIdMultiPartFormDataStream.Create;
try
PostStream.AddFormField('Ocp-Apim-Subscription-Key','YOUR_SUBSCRIPTION_KEY');
PostStream.AddFile('file', 'test.wav');
Response := HTTPClient.Post(url, PostStream);
PostStream.Clear;
finally
PostStream.Free;
end;
finally
HTTPClient.Free;
end;
end;

Answer 1

Your POST request is not setup the way Microsoft's documentation says. Most importantly, you should not be using TIdMultipartFormDataStream at all, since the REST server is not expecting a request in multipart/form-data format. The body of the request is expected to be just the actual WAV file and nothing else. TIdHTTP even has an overload of Post() specifically for uploading just a file.
Try this instead:
procedure TForm1.Button1Click(Sender: TObject);
var
Response, url: String;
HTTPClient: TIdHTTP;
SSL: TIdSSLIOHandlerSocketOpenSSL;
begin
url := 'https://speech.platform.bing.com/speech/recognition/interactive/cognitiveservices/v1?language=en-US&format=detailed';
HTTPClient := TIdHTTP.Create;
try
SSL := TIdSSLIOHandlerSocketOpenSSL.Create(HTTPClient);
SSL.SSLOptions.SSLVersions := [sslvTLSv1, sslvTLSv1_1, sslvTLSv1_2];
HTTPClient.IOHandler := SSL;
HTTPClient.AllowCookies := True;
HTTPClient.HandleRedirects := true;
HTTPClient.Request.Accept := 'application/json;text/xml';
HTTPClient.Request.ContentType := 'audio/wav; codec=audio/pcm; samplerate=16000';
HTTPClient.Request.CustomHeaders.Values['Ocp-Apim-Subscription-Key'] := 'YOUR_SUBSCRIPTION_KEY';
Response := HTTPClient.Post(url, 'test.wav');
finally
HTTPClient.Free;
end;
end;