Save Notepad++ File as SAIL File Type

Head to the user defined language options (Language -> Define your language). Select your language from the User language dropdown, and then add "sail" without the quotes to the box labelled "ext" (screenshot attached).

The first file you open with a .sail extension will require you to set the file associations (assuming windows); select notepad and check the "always open files of this type..." checkbox. Once it opens in notepad, it should open with the correct syntax highlighting.

I've also attached my version of the interface designer language in case you're interested.

interface_designer_udl.xml

If you select "Any Types (*.*)" you can save whatever file extension you want.

Also if you want to add the sail type to the context menu when saving a file, it's possible but involves a registry customisation, so I wouldn't necessarily recommend it.

I should also add that it's possible to branch Notepad++ if you wan to get really funky with the customisations: github.com/.../notepad-plus-plus

I've done (and personally recommend) exactly what @philb described in his first comment - this causes ".sail" files to open automatically with the user-defined language you choose (hopefully you have the SAIL77+ notepad extension installed already).

Additionally I'd like to point out that when saving a new/blank/unsaved file FROM NP++, you can just type "filename.sail" even if the type is set to .txt; NP++ automatically catches the overridden extension and uses the one you've asked for.