Download a file from a sharepoint document library using Sharepoint REST API

Hi There

I am trying to download a file from a document library using sharepoint REST API.

What I have done is.

1. I have created a console application in Visual Studio 2015

2. Have registered the App Principles and have generated a client id and Secret if using:-https://mysp.sharepoint.com/_layouts/15/appregnew.aspx

3. Also have given permission for the app using:- https://mysp.sharepoint.com/_layouts/15/appinv.aspx

Can some one please advice what should be the Permission Request XML at this stage.

4. In the console application I have added following nGuit packages

Strategik.CoreFramework.v16

5. The above package have created tokenhelper class for me.

6. I have added the clientid and Secret id in the config file

6. I have written following code in Main function

  String accessToken;
            Uri siteUri = new Uri("https://mysp.sharepoint.com/");
            string realm = TokenHelper.GetRealmFromTargetUrl(siteUri);
            accessToken = TokenHelper.GetAppOnlyAccessToken(TokenHelper.SharePointPrincipal ,siteUri.Authority,realm).AccessToken;

                  System.Net.HttpWebRequest endpointRequest = (HttpWebRequest)HttpWebRequest.Create(siteUri + "/_api/web/GetFolderByServerRelativeUrl('/foldername/filename')");
            endpointRequest.Method = "GET";
            endpointRequest.Accept = "application/json;odata=verbose";
            endpointRequest.Headers.Add("Authorization", "Bearer " + accessToken);
            endpointRequest.Headers.Add("X-RequestDigest", "form digest value");
            endpointRequest.Headers.Add("X-HTTP-Method", "PUT");
            //endpointRequest.Headers.Add("content-length", "length of post body");
            System.Net.HttpWebResponse endpointResponse = (System.Net.HttpWebResponse)endpointRequest.GetResponse();
            StreamReader reader = new StreamReader(endpointResponse.GetResponseStream());
            var searchXml = new XmlDocument();
            searchXml.LoadXml(reader.ReadToEnd());

7. I am getting error at

endpointResponse.GetResponseStream()

saying "The remote server returned an error: (403) Forbidden."

Can some one please help me. and advise if the above steps are correct. Please also advise for Point no3

Regards

Umesh