use 'file' command to make sure files have the proper (expected) extension

rename-files-by-filetype.sh

#!/usr/bin/env zsh -f
# Purpose: use 'file' command to make sure image files have proper extension
#
# From:	Timothy J. Luoma
# Mail:	luomat at gmail dot com
# Date:	2019-10-04

NAME="$0:t:r"

if [[ -e "$HOME/.path" ]]
then
	source "$HOME/.path"
else
	PATH='/usr/local/scripts:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin'
fi

for i in "$@"
do

	if [[ -f "$i" ]]
	then

			# make sure we have the full path to '$i'
		i=($i(:A))

		TYPE=$(file -b "$i" | awk '{print $1}')

		case "$TYPE" in
			JPEG)
						# make it lowercase
					EXPECTED_EXT='jpg'
			;;

			PNG)
						# make it lowercase
					EXPECTED_EXT='png'
			;;

			## If you want to add more filetypes, use `file -b "YourFileHere.ext" | awk '{print $1}'`
			## and see what you get as output. Then replace 'FOO' with it and 'foo' with the
			## proper extension.
			#
			# FOO)
			# 			# make it lowercase
			# 		EXPECTED_EXT='foo'
			# ;;

			*)
					echo "$NAME: '$i' is type '$TYPE' but I don't know what to do with that."
					continue
			;;
		esac

		ACTUAL_EXT="$i:e:l"

		if [[ "$EXPECTED_EXT" == "$ACTUAL_EXT" ]]
		then
			# the EXT that the file has is what it should have
			echo "	$NAME: '$i' has the proper extension."

			continue
		fi

			## if we get here, then the extension does not match what we
			## expected, which either means that it is a) missing or b) wrong

		if [[ "$ACTUAL_EXT" == "" ]]
		then
				## there is no extension, so we need to add one

			NEWNAME="$i.$EXPECTED_EXT"
		else
				## if we get here, the file has an extension
				## but it is not the correct one
			NEWNAME="$i:r.$EXPECTED_EXT"
		fi

			# initialize a counter
		COUNT='0'

		while [[ -e "$NEWNAME" ]]
		do
			# if there is another file with the same name we want to use
			# add a digit to the filename
			# so 'foo.jpg' would become 'foo.1.jpg'
			# this loop will repeat if 'foo.1.jpg' also exists

			((COUNT++))

			NEWNAME="$i:r.$COUNT.$EXPECTED_EXT"

		done

			# Once we get here we should be ready to rename the file
			# but we use `-n` just to be safe

		mv -vn "$i" "$NEWNAME"

	fi

done

exit 0
#EOF