Once you have written your file.service file, you have to copy it to
/lib/systemd/system, create a symbolic link to /etc/systemd/system and reload
the daemon:
cp file.service /lib/systemd/system/
ln -s /lib/systemd/system/file.service /etc/systemd/system/file.service
systemctl daemon-reload
After those steps, you can use your new service:
systemctl enable file.service
systemctl start file.service