customthumbs.sh

#!/bin/bash

target=$1
filename=`basename $1`
image="${TMPDIR}${filename}.png"
rsrc="${TMPDIR}icn.rsrc"

# Create a thumbnail from the file preview
qlmanage -t -s 512 -o ${TMPDIR} $target

# apply the image to itself as an thumbnail icon
sips -i $image

# Extract the icon resource
DeRez -only icns $image > $rsrc

# Make the target accept a custom icon
SetFile -a C $target

# Apply the icon resource to the target
Rez -append $rsrc -o $target

# clean up
rm $rsrc $image