.att is the extension used for the Creatures series ATT files, also known as body data files. These files can be viewed and edited in a plain text editor, such as Windows Notepad. They are used by the Creatures games to detail how to connect (or "attach") breed images together. Each breed image file has an associated attachment file by the same name, in which the co-ordinates of key points for each pose are written. You can find the file naming convention here.