Save as .sifz by default (if there is no '.' in the saved file's path).
drwxr-xr-x - ETL
drwxr-xr-x - autobuild
drwxr-xr-x - gtkmm-osx
drwxr-xr-x - synfig-core
drwxr-xr-x - synfig-docs
drwxr-xr-x - synfig-osx
drwxr-xr-x - synfig-studio