A couple of years ago Contiki had a lot of rough parts that were in dire need of a bit of love - something we've worked hard to fix in the past few versions. Version 3.0 is by far the most solid Contiki thus far. Maybe you got stuck in the middle of this

Not sure about Coffee in particular though, simply because file systems isn't really the focus of Contiki. Unlike server and desktop OSes, where the file system really is a core item, embedded OSes like Contiki don't depend on them and they are only used by a small subset of applications.