I use org-attach a lot. There's no way to see which files are attached without calling "org-attach-open" or whatever. Apparently something changed in Org-mode years ago that removed the function that did what I want. Found the following on the mailing list...

(defun org-attach-save-file-list-to-property (dir)
    "Save list of attachments to ORG_ATTACH_FILES property."
    (when-let* ((files (org-attach-file-list dir)))
      (org-set-property "ORG_ATTACH_FILES" (mapconcat #'identity files ", "))))
  (add-hook 'org-attach-after-change-hook #'org-attach-save-file-list-to-property)

Works great.