The callback system from LuaTeX has been disabled. Instead, Context defines actions that must be registered through an offical mechanism. Then they can be toggled on and off at will, as long as it makes sense.
This page is dedicated to collecting all available information on how to achieve in Context when you would resort to callbacks to do the same in plain LuaTeX.
File reading callbacks /
Hans demonstrated the canonical way on the mailing list :
\startluacode function document.MyCharacterMess(str,filename) if file.nameonly(filename) == "ward" then str = table.concat(string.totable(str,"."), " + ") end return str end local textfileactions = resolvers.openers.helpers.textfileactions utilities.sequencers.appendaction(textfileactions,"system","document.MyCharacterMess") \stopluacode \starttext \input ward \input knuth \stoptext