Upon further testing…
If I accept the default file name, the file saves with the extension.
If changing the name, it only uses an extension if I include one.
I assumed that it would append any file name I used with the correct extension.
It does not. The Windows UI “save file” window accepts [all files *] when saving.
As long as I manually add .pln to my custom file name it is good.
As an experienced user it was an easy find for me. I expect less computer savvy users could benefit from the default extension being auto appended. I believe this is an option in the API for the save file window.