What is the “#(filename)#” file that I see in my gallery?

That file is a backup file that was created due to the app forcibly closing/crashing

If your app crashes, it will appear as the newest file in your gallery so you can easily continue working on your latest piece.

NOTE: This backup file is temporarily saved as a measure against app crashes. If you open another file aside from the backup file after reopening the app, the backup file will be deleted.