The FileExtensions in the FileName is now a reference to the one in the original FileStyle, which gets put there in the options module.
This allows the extensions to be derived from the user, somehow, in the future when that part’s done.
Instead of having a File do its own extension checking, create a new type that takes a file and checks *that*. This new type (FileExtensions) is currently empty, but were it to contain values, those values could be used to determine the file’s colour.
The arguments passed to File’s constructor were different from the field names used — these might as well both be the same.
Also, move ext and filename to be File methods to save an import, and add tests.
Also also, by passing a PathBuf in to the constructor directly, we can save one (possibly two) instance/s where we pass in a reference to something we were going to lose ownership of anyway, only to have it basically cloned.
