diff --git a/src/gempub.lisp b/src/gempub.lisp
index e1c0971..be1cf27 100644
--- a/src/gempub.lisp
+++ b/src/gempub.lisp
@@ -71,6 +71,8 @@
 
 (define-constant +metadata-entry-name+ "metadata.txt" :test #'string=)
 
+(define-constant +gempub-file-extension+ "gpub" :test #'string=)
+
 (defun extract-metadata (zip-file)
   (when (gempub:gempub-file-p zip-file :ignore-errors t)
     (let ((entries (zip-info:list-entries zip-file)))
diff --git a/src/gui/client/gempub-window.lisp b/src/gui/client/gempub-window.lisp
index bd6f4c3..7cf97cb 100644
--- a/src/gui/client/gempub-window.lisp
+++ b/src/gui/client/gempub-window.lisp
@@ -181,7 +181,9 @@
               license
               copyright
               cover-filename))
-    (os-utils:zip-directory gemtext-directory :flat t)))
+    (os-utils:zip-directory gemtext-directory
+                            :extension gempub:+gempub-file-extension+
+                            :flat t)))
 
 (defun create-gempub (gemtext-dir)
   (gui:with-modal-toplevel (toplevel :title (_ "Add Metadata"))
diff --git a/src/os-utils.lisp b/src/os-utils.lisp
index 3a5a15a..b28e4bf 100644
--- a/src/os-utils.lisp
+++ b/src/os-utils.lisp
@@ -319,13 +319,16 @@ numerical user ID, as an assoc-list."
       (when (not (process-exit-success-p process))
         (error (format nil (_ "File ~s extraction from ~s failed") file-entry zip-file))))))
 
-(defun zip-directory (dir &key (flat nil))
+(defun zip-directory (dir &key (extension nil) (flat nil))
   (with-output-to-string (stream)
     (let* ((process (run-external-program +zip-bin+
                                           (append (when flat
                                                     (list "-j"))
                                                   (list "-r"
-                                                        dir
+                                                        (format nil
+                                                                "~a~@[.~a~]"
+                                                                dir
+                                                                extension)
                                                         dir))
                                           :search t
                                           :wait   t
diff --git a/src/package.lisp b/src/package.lisp
index 2ad9abd..0b4e66e 100644
--- a/src/package.lisp
+++ b/src/package.lisp
@@ -2946,6 +2946,7 @@
   (:shadowing-import-from :misc :random-elt :shuffle)
   (:export
    :+metadata-entry-name+
+   :+gempub-file-extension+
    :gempub-file-p
    :extract-metadata
    :sync-library