I installed the Grails zipped-resources plugin on a new project the other day, and I noticed that by default it zips up everything it sends.
This is fine, but when I tried to make it ignore the image files being served up, it wasn't obvious at first how to do that. Finally, I figured out what I needed to do. Adding the following line to my Config.groovy excluded the image files from being zipped up.
grails.resources.mappers.zip.excludes = ['**/*.png','**/*.gif','**/*.jpg','**/*.jpeg','**/*.gz','**/*.zip']
Once I read more about defining a mapper, it was nice to see that even configuration of these resource files was taking on a convention.
That said, the suite of resources plugins that are coming out now for Grails are very useful. I would say that they really help make Grails a strong contender in platform choices for the future.