Download file with Angular and Filesaver.js not working

i use Filesaver.js to try to download files from my backend, which is on Express (nodejs).

Here is my backend code to download files. It uses a middleware to check the auth info.

res.download(url, function (err) {
   if (err) {
      console.log("Error: " + err);
      res.status.send({
         message: "Can not download file"
      });
   }
});

And here is the code of my service:

downloadEventAttachment(attachmentUrl){

    let endPointUrl = this.auth.getServerHost();

    this.authHttp.get(endPointUrl + attachmentUrl)
        .subscribe(res => {
            var blob = new Blob([res], { type: res.headers.get('Content-Type') });
            saveAs(blob);
        }) 
}

The problem is that generates a download file but with errors. If i download a image it says that the image is not valid.

If i try to download a txt file its have this content:

Response with status: 200 OK for URL: http://192.168.1.78:8081/calendar/download_eventattachment/728/file-1496421767591.txt

If i use this request from postman, then the image is well displayed.

I'm doing something wrong?

Thanks a lot

via Ali Briceño