diff --git a/scrapy/pipelines/files.py b/scrapy/pipelines/files.py
index f9dfa53e3..94b0b1b70 100644
--- a/scrapy/pipelines/files.py
+++ b/scrapy/pipelines/files.py
@@ -37,7 +37,7 @@ from scrapy.utils.request import referer_str
 logger = logging.getLogger(__name__)
 
 
-def _to_string(path: Union[str, PathLike]):
+def _to_string(path: Union[str, PathLike]) -> str:
     return str(path)  # convert a Path object to string