I want to download a file that it's API service created in php7. when I receive service output from API service, the file doesn't download. But if I see service output in developer tools of browser it's correct. What is wrong in my app? 
To get API service in angular I use HttpClient of angular2 & get method. after subscribe the http service result is not empty but not download file. How can I use stream downloading in angular2?
downloadAttachment(filename: string){
   return this.http.get( this.url +"/"+ filename)
            .catch(this.handleError);
}