No way to set `#+FILETAGS` programmatically.

[broken-pen]

As suggested by @kristijanhusak in #878 (comment), this issue is to remind us that there currently is no way to add or remove file-global tags in the #+FILETAGS directive.

This is a bother because org-attach has an auto-tag feature that it wants to both use on headlines (if the cursor is under a headline) and on files as a whole (if the cursor is above the first headline).

FWIW, this is also an issue in the current version of Emacs org-attach, see https://github.com/emacs-mirror/emacs/blob/492adf20b91520e96fb198e6e936e3145359c43b/lisp/org/org-attach.el#L532.