I don't know where you that very brief explanation, but I would suggest the avast Help file, Resident Protection, Standard Shield, Blocker Page. As far as dangerous extensions go, .exe, .com, .dll, .sys, etc. typically files that are executable. For a full list of the default set check Standard Shield, Customise, Blocker, Default extension set click Show...
Personally I don't have any of these enabled (which is the default setting), you could add open file for writing and try to edit a file and see the response.