#!/bin/bash

# lowerext.sh    

while read f; do
    if [[ "$f" = *.* ]]; then
        # Extract the basename
        b="${f%.*}"

        # Extract the extension
        x="${f##*.}"

        # Convert the extension to lower case
        # Note: this only works in recent versions of Bash
        l="${x,,}"

        if [[ "$x" != "$l" ]]; then
            mv "$f" "$b.$l"
        fi
    else
        continue
    fi
done