There are several ways you can improve upon this app:
Our app only asks the user if they really want to delete a file, but other operations are equally dangerous. Add in confirmations if the action the user is about to perform would overwrite data (for example, renaming a document to an existing document's name or creating a document with the same name as an existing document).
Add functionality to check if the filename a user is supplying is valid. Then, either indicate this to the user or silently change the invalid characters to valid characters.
We don't provide subdirectory functionality, but there's no reason why you couldn't. In fact, we explicitly ignore subdirectories in our code, as they add a lot of complexity to the file management system. Why don't you add subdirectory management to the app?
Instead of storing notes, perhaps you could store some forms instead. Perhaps simple addresses or reminders—really, just about anything.