added to config:
int send_file_mode;
// 0 - full path to a file in send_file_header header
// 1 - relative path to a file in send_file_header (need http_send_file_relative_prefix set) (used for nginx)
std::wstring send_file_header;
// default: X-SENDFILE
// for Apache set: X-SENDFILE
// for Lighttpd set: X-LIGHTTPD-send-file
// for Nginx set: X-Accel-Redirect
std::wstring send_file_relative_prefix;
// relative prefix used for sending static files if send_file_mode is 1
// default: "upload-files-internal"