diff --git a/lisp/files.el b/lisp/files.el
index 62e1702fdf..18a54b3ec7 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -4889,6 +4889,20 @@ extension, the value is \"\"."
         (if period
             "")))))
 
+(defun file-name-set-extension (filename extension)
+  "Change the extension of a FILENAME to EXTENSION.
+Sanitizes the input to consolidate leading/trailing dots.
+
+Returns `nil' if either of the FILENAME or EXTENSION are `nil'
+before sanitizing, or empty afterwards."
+  (when (and filename extension)
+    (let* ((patt "[ \t\n\r.]+") ; Inspired by `string-trim'.
+           (filename (string-trim-right filename patt))
+           (extension (string-trim-left extension patt)))
+      (unless (or (string-empty-p filename)
+                  (string-empty-p extension))
+        (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"))