diff --git a/lisp/files.el b/lisp/files.el
index 62e1702fdf..eed818e2fa 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -4889,6 +4889,15 @@ extension, the value is \"\"."
         (if period
             "")))))
 
+(defun file-name-set-extension (filename extension)
+  "Set the EXTENSION of a FILENAME.
+Consolidates leading/trailing dots so that either `foo' or `.foo'
+can be passed as an EXTENSION."
+  (let* ((patt "[ .]+")
+         (filename (string-trim-right filename patt))
+         (extension (string-trim-left extension patt)))
+    (concat (file-name-sans-extension filename) "." extension)))
+
 (defun file-name-base (&optional filename)
   "Return the base name of the FILENAME: no directory, no extension."
   (declare (advertised-calling-convention (filename) "27.1"))