diff --git a/backend/pashub_fetcher/core_files.py b/backend/pashub_fetcher/core_files.py
index 30aa2ba8..e63511eb 100644
--- a/backend/pashub_fetcher/core_files.py
+++ b/backend/pashub_fetcher/core_files.py
@@ -69,7 +69,9 @@ def get_core_file_type(
 
 
 def get_file_type_string(filename: str) -> Optional[str]:
-    core_file = get_core_file_type(filename)
+    core_file: Optional[CoreFiles] = get_core_file_type(filename)
+
     if core_file is None:
         return None
+
     return _CORE_FILE_TO_FILE_TYPE[core_file]