Feishu's GetMessageResource API only accepts 'image' or 'file' as the
type parameter. When downloading voice messages, nanobot was passing
'audio' which caused the API to reject the request with an error.
This fix converts 'audio' to 'file' in _download_file_sync method
before making the API call, allowing voice messages to be downloaded
and transcribed successfully.
Fixes voice message download failure in Feishu channel.