reverse is a
chIdx is 0
a.png -> b.png
reverse is z
chIdx is 0
z.jpg -> aa.jpg
reverse is za
chIdx is 0
chIdx is 1
az.png -> ba.png
reverse is zaz
chIdx is 0
chIdx is 1
zaz.png -> zba.png
ott.jpg -> ptt.jpg
reverse is zzz
chIdx is 0
chIdx is 1
chIdx is 2
zzz.tiff -> aaaa.tiff
oi.png -> pj.png
reverse is cba
chIdx is 0
abc.jpg -> abd.jpg
$