Download speed can depend on the peering agreement between your hosting ISP and the ISP your customer uses for downloading the file. First step should be asking your customer to use traceroute to identify bottlenecks in the connection.
Google provides a service to speed test your server. See here. Unfortunately Turkey is not one of their testing locations.
If your HTTP server is running Apache the best testing method would be using the Apache Module mod_log_config to write a custom logfile. The option %T (case sensitive. Capital T!) measures "The time taken to serve the request, in seconds."