diff --git a/docs/handbook/writing-your-own-file-decoder.rst b/docs/handbook/writing-your-own-file-decoder.rst
index 24e46ff00..4be34eefc 100644
--- a/docs/handbook/writing-your-own-file-decoder.rst
+++ b/docs/handbook/writing-your-own-file-decoder.rst
@@ -53,6 +53,11 @@ true color.
 
     from PIL import Image, ImageFile
 
+
+    def _accept(prefix):
+        return prefix[:4] == b"SPAM"
+
+
     class SpamImageFile(ImageFile.ImageFile):
 
         format = "SPAM"
@@ -60,12 +65,7 @@ true color.
 
         def _open(self):
 
-            # check header
-            header = self.fp.read(128)
-            if header[:4] != b"SPAM":
-                raise SyntaxError("not a SPAM file")
-
-            header = header.split()
+            header = self.fp.read(128).split()
 
             # size in pixels (width, height)
             self._size = int(header[1]), int(header[2])
@@ -86,7 +86,7 @@ true color.
                 ("raw", (0, 0) + self.size, 128, (self.mode, 0, 1))
             ]
 
-    Image.register_open(SpamImageFile.format, SpamImageFile)
+    Image.register_open(SpamImageFile.format, SpamImageFile, _accept)
 
     Image.register_extension(SpamImageFile.format, ".spam")
     Image.register_extension(SpamImageFile.format, ".spa") # dos version