Xman,
Please follow these steps:
1 Open your service on settings tab.
2 Use as URL following string:
pre
{database_url}/files/{file_name}
/pre
http://prntscr.com/46456m/direct
3 Navigate to request tab.
4 Add new request parameter "file_name".
http://prntscr.com/4645uo/direct
Now you have "request" parameter "file_name" which is you can modify as you need in mapping.
Regards