Bash script to create .m3u playlist files for all mp3s in subdirectories

playlist.sh

#!/bin/bash
#
# bash script to create playlist files in music subdirectories
#
# Steve Carlson (stevengcarlson@gmail.com)

find . -type d |
while read -r subdir
do
  rm -f "$subdir"/*.m3u
  for filename in "$subdir"/*
  do
    if [ "${filename: -4}" == ".mp3" ] || [ "${filename: -5}" == ".flac" ] || [ "${filename: -5}" == ".loss" ] || [ "${filename: -5}" == ".aiff" ] || [ "${filename: -4}" == ".aif" ]
    then
        echo "${filename##*/}" >> ./"$subdir"/"${subdir##*/}.m3u"
    fi
  done

done

I still get "playlist.sh: 13 playlist.sh: Bad substitution" error on RaspianOS with NTFS drives. Have no idea why. :(