I don't believe it can. There was some back-and-forth in the Samba mailing list about introducing a "allow files" parameter, but nothing ever came of it. Your best bet is to write a maintenance script that removes anything that doesn't have a proper file extension. That said, file extension != file type. It might not take a long time for users to find out they can add ".xslx" to their cat videos and store them on your server, and so you might want something a bit better than a simple regex based on file names.