Raise SyntaxError if data is not as expected

Tests/test_file_xbm.py

@@ -2,7 +2,7 @@ from io import BytesIO
 
 import pytest
 
-from PIL import Image
+from PIL import Image, XbmImagePlugin
 
 from .helper import hopper
 
@@ -63,6 +63,13 @@ def test_open_filename_with_underscore():
         assert im.size == (128, 128)
 
 
+def test_invalid_file():
+    invalid_file = "Tests/images/flower.jpg"
+
+    with pytest.raises(SyntaxError):
+        XbmImagePlugin.XbmImageFile(invalid_file)
+
+
 def test_save_wrong_mode(tmp_path):
     im = hopper()
     out = str(tmp_path / "temp.xbm")

src/PIL/XbmImagePlugin.py

@@ -52,7 +52,8 @@ class XbmImageFile(ImageFile.ImageFile):
 
         m = xbm_head.match(self.fp.read(512))
 
-        if m:
+        if not m:
+            raise SyntaxError("not a XBM file")
 
         xsize = int(m.group("width"))
         ysize = int(m.group("height"))