diff --git a/api/core/datasource/utils/message_transformer.py b/api/core/datasource/utils/message_transformer.py
index bfdd5c214d..bb6ac6c1fc 100644
--- a/api/core/datasource/utils/message_transformer.py
+++ b/api/core/datasource/utils/message_transformer.py
@@ -62,7 +62,7 @@ class DatasourceFileMessageTransformer:
 
                 mimetype = meta.get("mime_type")
                 if not mimetype:
-                    mimetype = guess_type(filename)[0] or "application/octet-stream"  # pyright: ignore[reportArgumentType]
+                    mimetype = (guess_type(filename)[0] if filename else None) or "application/octet-stream"
 
                 # if message is str, encode it to bytes