Thanks for your question.
Unfortunately, Message Viewer relies on the square brackets for extracting information that is encoded between the brackets. The information is required for displaying the correct icons for a given message in the leftmost column of the message list.
Message Saver does not provide a setting to exclude the square brackets. If you remove the brackets, the Viewer will display incorrect icons in the message list and all items will appear as Unread. However everything else should work, so long as the message file’s extension remains “.fml”.