The file type extension of the file is yielded by this property. In the past, extensions conformed to a three character standard. With the cross mounting of foreign file systems which did not conform to the DOS 8.3 standard, file extensions can be of any length although they are usually less than five characters long.
It is normal to hide the file extensions from the users of desktop systems. In that case you may want to hold the filename and extension separately, so that you can hide the file extension to be consistent with what your users see on the desktop.