UPDATE: Looks like Apple’s re-issue of Security Update 2017-001 most likely addressed the file sharing issue, so there’s nothing you need to do on your end. [H/T, Bryan Lee]
If file sharing stops working, go to this Apple Support knowledge base article.
Short story even shorter, you’ll go to Terminal and enter this line:
Follow with your admin password, when prompted, and you should be good to go. This is all a bit of a mess, but kudos to Apple for their rapid response here.