diff --git a/bdfr/resource.py b/bdfr/resource.py
index ddad278..12c9cc4 100644
--- a/bdfr/resource.py
+++ b/bdfr/resource.py
@@ -63,7 +63,7 @@ class Resource:
     def create_hash(self):
         self.hash = hashlib.md5(self.content)
 
-    def _determine_extension(self) -> str:
+    def _determine_extension(self) -> Optional[str]:
         extension_pattern = re.compile(r'.*(\..{3,5})(?:\?.*)?$')
         match = re.search(extension_pattern, self.url)
         if match: