[ Sorry for the slow reply. ]

Simon Braß writes:

> Hi all,
>
> I've tried to open an attached file (added with org-attach-attach), however,
> when I try to open it with C-c C-o I got the following backtrace:
>
> #+begin_example
> Debugger entered--Lisp error: (wrong-number-of-arguments (2 . 2) 1)
>   org-attach-follow("1805.00020.pdf")
>   org-link-open((link (:type "attachment" :path "1805.00020.pdf" :format
> bracket :raw-link "attachment:1805.00020.pdf" :application nil
> :search-option nil :begin 717 :end 818 :contents-begin 746 :contents-end
> 816 :post-blank 0 :parent (paragraph (:begin 717 :end 819 :contents-begin
> 717 :contents-end 819 :post-blank 0 :post-affiliated 717 :parent (item
> (:bullet "- " :begin 708 :end 819 :contents-begin 717 :contents-end 819
> :checkbox off :counter nil :structure (... ... ... ... ...) :pre-blank 0
> :post-blank 0 :post-affiliated 708 :tag nil :parent (plain-list ...)))))))
> nil)
>   org-open-at-point(nil)
>   funcall-interactively(org-open-at-point nil)
>   call-interactively(org-open-at-point nil nil)
>   command-execute(org-open-at-point)
> #+end_example

Hmm, this looks like an inaccessible state for Org 9.4.4.  Here's how
org-link-open would handle an attachment type:

  (let ((f (org-link-get-parameter type :follow)))
    (when (functionp f)
      ;; Function defined in `:follow' parameter may use a single
      ;; argument, as it was mandatory before Org 9.4.  This is
      ;; deprecated, but support it for now.
      (condition-case nil
          (funcall (org-link-get-parameter type :follow) path arg)
        (wrong-number-of-arguments
         (funcall (org-link-get-parameter type :follow) path)))))

So it first calls with the expected 2 arguments and then falls back to
the single one.

Follow attachment links works okay on my end, and, aside from a mixed
installation, I can't think of a way to get into that state.

  https://orgmode.org/worg/org-faq.html#mixed-install

> After adding =&optional= to the definition of =org-attach-follow=, and
> reloading org-mode (uncompiled) I could get through org-attach-follow.
> But, then I hit another issue:
>
> #+begin_example
> Debugger entered--Lisp error: (void-function org-link-open-as-file)
>   (org-link-open-as-file (org-attach-expand file) arg)
>   org-attach-follow("0906.4487.pdf")
>   …
> #+end_example

That function was added in 9.4 and is in the same file as the caller,
org-link-open.  The fact that it's not defined also points to a mixed
install, with an older ol.el taking precedence.