diff --git a/Tests/test_file_spider.py b/Tests/test_file_spider.py
index 3b3c3b4a5..03494523b 100644
--- a/Tests/test_file_spider.py
+++ b/Tests/test_file_spider.py
@@ -14,6 +14,10 @@ from .helper import assert_image_equal, hopper, is_pypy
 TEST_FILE = "Tests/images/hopper.spider"
 
 
+def teardown_module() -> None:
+    del Image.EXTENSION[".spider"]
+
+
 def test_sanity() -> None:
     with Image.open(TEST_FILE) as im:
         im.load()
diff --git a/Tests/test_image.py b/Tests/test_image.py
index afc6e8e16..c064939e8 100644
--- a/Tests/test_image.py
+++ b/Tests/test_image.py
@@ -466,6 +466,9 @@ class TestImage:
         # Assert
         assert Image._initialized == 2
 
+        for extension in Image.EXTENSION:
+            assert extension in Image._EXTENSION_PLUGIN
+
     def test_registered_extensions(self) -> None:
         # Arrange
         # Open an image to trigger plugin registration
diff --git a/src/PIL/SpiderImagePlugin.py b/src/PIL/SpiderImagePlugin.py
index 866292243..848dccda5 100644
--- a/src/PIL/SpiderImagePlugin.py
+++ b/src/PIL/SpiderImagePlugin.py
@@ -290,9 +290,9 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
 
 def _save_spider(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
     # get the filename extension and register it with Image
-    filename_ext = os.path.splitext(filename)[1]
-    ext = filename_ext.decode() if isinstance(filename_ext, bytes) else filename_ext
-    Image.register_extension(SpiderImageFile.format, ext)
+    if filename_ext := os.path.splitext(filename)[1]:
+        ext = filename_ext.decode() if isinstance(filename_ext, bytes) else filename_ext
+        Image.register_extension(SpiderImageFile.format, ext)
     _save(im, fp, filename)