[Rspamd] Disable extension cloaking (#3754)

data/conf/rspamd/local.d/mime_types_group.conf

@@ -0,0 +1,7 @@
+symbols = {
+    "MIME_DOUBLE_BAD_EXTENSION" {
+        weight = 0; # This rule has dynamic weight up to 4.0
+        description = "Bad extension cloaking";
+        one_shot = true;
+    }
+}