Avoid always-false statement

See https://github.com/pallets/flask/pull/1849/files#r79371299

flask/helpers.py

@@ -559,7 +559,7 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
         rv.cache_control.max_age = cache_timeout
         rv.expires = int(time() + cache_timeout)
 
-    if add_etags and filename is not None and file is None:
+    if add_etags and filename is not None:
         from warnings import warn
 
         try: