bug#39994: 27.0.90; Broken image-converter probe for imagemagick

[Juri Linkov]

> diff --git a/lisp/image/image-converter.el b/lisp/image/image-converter.el
> index 0488a13d41..4efae5c202 100644
> --- a/lisp/image/image-converter.el
> +++ b/lisp/image/image-converter.el
> @@ -57,6 +57,10 @@ image-convert-p
>    ;; Find an installed image converter.
>    (unless image-converter
>      (image-converter--find-converter))
> +  ;; When image-converter was customized
> +  (if (and image-converter (not image-converter-regexp))
> +      (when-let ((formats (image-converter--probe image-converter)))
> +        (setq image-converter-regexp (concat "\\." (regexp-opt formats) 
> "\\'"))))
>    (and image-converter
>         (or (and (not data-p)
>                  (string-match image-converter-regexp source))

Pushed this fix to emacs-27.

Other (apparently more minor) problems could be fixed later.