<P> Around this same time appeared the dot (period or full - stop) as a filename extension separator, and the limit to three letter extensions might have come from 16 - bit RAD50 character encoding limits . </P> <P> Traditionally, most operating system supported filenames with only uppercase alphanumeric characters, but as time progressed, the number of characters allowed increased . This led to compatibility problems when moving files between different file systems . </P> <P> In 1985, RFC 959 officially defined a pathname to be the character string that must be entered into a file system by a user in order to identify a file . </P> <P> Around 1995, VFAT, an extension to the MS - DOS FAT filesystem, was introduced in Windows 95 and Windows NT . It allowed mixed - case Unicode long filenames (LFNs), in addition to classic "8.3" names . </P>

A file type is an optional file identifier that is separated from the main file name by a period