(defconst my/cygwin-p (string-equal system-type "cygwin"))
(defconst my/age-pubkey "ageXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX")

(defun my/dwim-shell-command/convert-path-cygwin (template unix-path)
  "Convert a Unix path to a Windows path.

This can be used as a :post-process-template of a
dwim-shell-command-on-marked-files function, where <<f>>
templates expand to Unix-like paths. However, a non-Cygwin binary
does not understand Unix paths so use cygpath to convert it."
  (if my/cygwin-p
      (let* ((command (format "cygpath -w %s" unix-path))
             (cygwin-path (string-trim (shell-command-to-string command)))
             (escaped-cygwin-path (string-replace "\\" "\\\\" cygwin-path)))
        (string-replace unix-path escaped-cygwin-path template))

    ;; just return the template as-is
    template))

(defun my/dwim-shell-command/age-encrypt ()
  "Encrypt marked files with the age encryption tool."
  (interactive)
  (dwim-shell-command-on-marked-files "Age Encypt"
                                      (format "age -e -r %s -o <<f>>.age <<f>>" my/age-pubkey)
                                      :utils "age"
                                      :post-process-template #'my/dwim-shell-command/convert-path-cygwin))