replace message content type list to string when file_objs is empty .. (#2745)

api/core/memory/token_buffer_memory.py

@@ -47,6 +47,9 @@ class TokenBufferMemory:
                     files, message.app_model_config
                 )
 
+                if not file_objs:
+                    prompt_messages.append(UserPromptMessage(content=message.query))
+                else:
                     prompt_message_contents = [TextPromptMessageContent(data=message.query)]
                     for file_obj in file_objs:
                         prompt_message_contents.append(file_obj.prompt_message_content)