diff --git a/lisp/image.el b/lisp/image.el
index 6a8293fedf3..104fea407ff 100644
--- a/lisp/image.el
+++ b/lisp/image.el
@@ -51,7 +51,7 @@ static \\(unsigned \\)?char \\1_bits" . xbm)
     ("\\`\\(?:MM\0\\*\\|II\\*\0\\)" . tiff)
     ("\\`[\t\n\r ]*%!PS" . postscript)
     ("\\`\xff\xd8" . jpeg)    ; used to be (image-jpeg-p . jpeg)
-    ("\\`RIFF....WEBPVP8" . webp)
+    ("\\`RIFF[^z-a][^z-a][^z-a][^z-a]WEBPVP8" . webp)
     (,(let* ((incomment-re "\\(?:[^-]\\|-[^-]\\)")
 	     (comment-re (concat "\\(?:!--" incomment-re "*-->[ \t\r\n]*<\\)")))
 	(concat "\\(?:<\\?xml[ \t\r\n]+[^>]*>\\)?[ \t\r\n]*<"