I assume this is a bug — I don't see how it would be an improvement — but in both 10.13.1 and now 10.13.2, I can't add file extensions in the Finder, at least not for JPG or PDF. Every time I try to edit the file name for a saved file, the extension is deleted after I click "return." (I have "show all file extensions" turned off, as I have for years, but before 10.13.1, I was always able to add a file name when I needed to.)