Rename all files extensions to lowercase with zsh, ignoring directories.

zmv

Rename all files extensions to lowercase with zsh, ignoring directories.

#load zmv
autoload -U zmv

zmv -Qvn '(**/)(*).(*)(.D)' '$1$2.${(L)3}'

Whats happening here:
  -Q enable globs (for directory matching later on)
  -vn verbose output, no executuion - remove these after checking this command does what you want it to
  (**/) go through all subdirectories, target saved in $1
  (*).(*) filename up to last point - $2 '.' filextension after last point - $3
  (.D) ignore directories
  ${(L)3} make $3 lowercase

Since $3 will only target stuff after the last dot, this wont work for double extenstions like .TAR.GZ (would become .TAR.gz)