Fix issue with file validation in Message class

src/backend/base/langflow/schema/message.py

@@ -37,6 +37,8 @@ class Message(Data):
     def validate_files(cls, value):
         if not value:
             value = []
+        elif not isinstance(value, list):
+            value = [value]
         return value
 
     def model_post_init(self, __context: Any) -> None:
@@ -174,3 +176,4 @@ class Message(Data):
         instance.prompt = jsonable_encoder(prompt_template.to_json())
         instance.messages = instance.prompt.get("kwargs", {}).get("messages", [])
         return instance
+        return instance