m3u and pls are the extensions associated with playlists as implemented
by winamp. Here [http://forums.winamp.com/showthread.php?threadid=65772]
is an unofficial definition.

----

A snippet to create a file in m3u and pls. It seems to
work with winamp 5.24, so I guess it's correct :)

    switch -- $form {
        m3u {
            set output "#EXTM3U"
            foreach l [array names tracks] {
                append output [format "#EXTINF:%d,%s\n%s\n" \
                  $tracks(length) $tracks(title) $tracks(location)
            }
        }
        pls {
            set output "\[playlist\]\n"
            set count 1
            foreach l [array names tracks] {
                append output [format "File%d=%s\n" $count $tracks(location)]
                append output [format "Title%d=%s\n" $count $tracks(title)]
                append output [format "Length%d=%d\n" $count $tracks(length)]
                incr count
            }
            append output [format "NumberOfEntries=%d\nVersion=2\n" \
              $count ]
        }
        default {
            error "unknown file format"
        }
    }
    set fh [open $path w]
    puts -nonewline $fh $output
    close $fh