Came across this on Pro Git 7.2 Customizing Git Git Attributes
You can use git attributes to allow for customized diff of binary files.
My current additions to my global git config are
git config --global diff.strings.textconv strings git config --global diff.exif.textconv exiftool
Then in projects (either .gitattributes or .git/info/attributes depending on whether you want it checked in or not):
*.<ext> diff = <diffname>
*.png diff = exif *.jpg diff = exif *.doc diff = strings