Invoice is not attached to email due to problem to access private filesystem from phpmailer

This is a report from 2022, I don't know if this is still accurate as I tested generating an invoice just now and correctly got the attachment via email...

I'm not sure the change is fully necessary, however if it fixes your issue and doesn't break the current functionality I'm open to committing the change.

Ok I made the change anyway, opened an MR, let's see if the tests are passing.