Can anyone tell me how I can tell Filemaker to start and open a
specific file? Like to do this one a regular interval via cron for
example.
It depends on the method you are using to serve the files.
Presuming you are running FMP server and have FMP clients. The method
is to run an "opener" file, an FMP database which has a single script
which runs on opening. The script requests a file from the server, then
closes itself (the opener file).