I have just received this from Garmin. It is the first refence I have seen to the .fpl file extension:
"The file must be created and saved as a .fpl file. The only programs that we are aware that have this capability are the Seattle Avionics Voyager, Jeppesen Flite Star and fltplan.com software products.
Thank you and best regards,
Aaron S.
Aviation Product Support Supervisor
Aviation Portable and Pilot Operations
1200 E. 151st Street | Olathe, KS 66062, USA | Phone: +1.866.739.5687 | email: [email protected]"