If you're willing/able to take Unix tools to your Windows machine (which I guess you are, since you're using du already), you could try using findutils from cygwin in a bash script:
for filetype in doc xls jpg mov; do
combined=0
find /cygdrive/c -name "*.$filetype" -type f -print0 | xargs -0 du -k | \
(while read; do
current=$(echo $REPLY | awk '{print $1}'
combined=$(( $combined + $current ))
done)
echo "$filetype files use $combined KiB on C:"
done
NB - this is roughly what I'd do; it's not tested.
Surely, this could be done more efficiently if you would like to do this for every type of file in the system. For single types, it probably wouldn't get much better with dedicated tools since this should be so IO-bound.