Call qlmanage and return focus to Emacs.

example.el

(require 'dired)

(defun qlmanage-return-focus-to-emacs ()
"Doc-string."
  (let* ((filename (dired-get-file-for-visit))
         (emacs-executable (concat invocation-directory "Emacs"))
         (emacs-script (concat "tell application \"" emacs-executable "\" to activate")))
      (set-process-sentinel
        (start-process "qlmanage" nil "/usr/bin/qlmanage" "-p" filename)
        `(lambda (p e) (when (= 0 (process-exit-status p))
          (set-process-sentinel
            (start-process "emacs" nil "osascript" "-e" ,emacs-script)
            '(lambda (p e) (when (= 0 (process-exit-status p))
              (message "Congratulations:  %s" ,filename)))))))))