Ok I commited your changes with some corrections from my previous post,
see below.

Philip Kaludercic <philipk@posteo.net> writes:

> Thierry Volpiatto <thievol@posteo.net> writes:
>
>> Philip Kaludercic <philipk@posteo.net> writes:
>    (let ((beg (line-beginning-position))
>          (end (point)))
> +    ;; Does it make sense to extend beyond END to allow completing
> +    ;; file names mid-string?
>      (list beg end #'completion-file-name-table
>            :exit-function (lambda (str _status)
>                               (when (and (stringp str)
>                                          (eq (char-after) ?/))
>                                 (delete-char -1))))))

Yes, we want to complete from bol to point:

     START and END delimit the text to complete (which should enclose
     point).  COLLECTION is a completion table for completing that text,

> -      (let ((backup-by-copying t))
> +      (let ((backup-by-copying t))	;why is this bound?

We want to backup by copying, not renaming, so ensure it is done by copying.

-- 
Thierry