> It will not work.  This is because of this form in mm-save-part:
>     (if (file-directory-p file)
>         (setq file (expand-file-name filename file))
> (file-directory-p file) is non-nil, filename is nil.

Confirmed.  What do you think the best solution?
(error "No file name")
(message "File name is required")
(while (not (setq file (read-file-name "Save MIME part to..." ...))))