Add check_media_file_for_spam spam checker hook

docs/spam_checker.md

@@ -61,6 +61,9 @@ class ExampleSpamChecker:
 
     async def check_registration_for_spam(self, email_threepid, username, request_info):
         return RegistrationBehaviour.ALLOW  # allow all registrations
+
+    async def check_media_file_for_spam(self, file_wrapper, file_info):
+        return False  # allow all media
 ```
 
 ## Configuration