Avoiding massive controller: where to put individual files

The question is not so much how I alter the URL mapping, but rather where do these files go in the directory structure of CI? They are not controllers. Do they belong in the library directory? Or core directory?

core is to extend or replace core libraries.
model is for your data model (database classes).
libraries is for your other classes.
helpers is for file containing functions.
third_party is for files from other project used by you application.