refactor(core): simplify role handling and improve usability

- Replaced explicit string usage with `CreatedByRole` enum for better maintainability.
- Removed duplicate `CreatedByRole` class definition, improving codebase consistency.
- Increased file number limits from 6 to 10 to allow more file uploads.
- Transitioned `AppMode` to a string enum for consistent type usage.
- Refactored `extract_thread_messages` function argument for flexibility.
- Removed file extension limitation in file service to support custom extensions.
- Improved enum import statements across multiple modules for clarity and consistency.

api/core/memory/token_buffer_memory.py

@@ -72,7 +72,7 @@ class TokenBufferMemory:
             files = db.session.query(MessageFile).filter(MessageFile.message_id == message.id).all()
             if files:
                 file_extra_config = None
-                if self.conversation.mode not in {AppMode.ADVANCED_CHAT.value, AppMode.WORKFLOW.value}:
+                if self.conversation.mode not in {AppMode.ADVANCED_CHAT, AppMode.WORKFLOW}:
                     file_extra_config = FileUploadConfigManager.convert(self.conversation.model_config)
                 else:
                     if message.workflow_run_id: