From the online manual
The Default path defines a relative or absolute path, that is to be used as the starting point in the Open/Save dialogs. When an absolute path is specified, it is used unchanged in all cases.
When no path is specified as default path, the current working directory is used, if it contains files, which satisfy the Filter strings below. Otherwise, the personal files directory of the current user is used.
When a relative path is specified (e.g. . or \docs), it always refers to the current working directory. Note: The working directory is usually a directory, where TED Notepad is started from. Then this working directory changes with every Open/Save dialogs operation.
Tip: A useful example for portable usage is a simple back-slash (\), which will make the Open/Save dialogs start in the root directory of the drive, where you launched the application. This can be used to twist your USB drive, when moving TED Notepad on the USB stick drive from one computer to another."
(Look under Options --> Settings --> File tab --> "Default path" field)
Sounds like you would want to paste ".\files" (without quotes) in the "Default path" field.
P.S. Good idea!