All artwork/ files and mechanical material is the property of the originator ( ie You ) They have no legal claim or ownership over the files. The contract was fhe production of a pack design etc. That contract was fullfilled. Many clients think that they own everything that was produced, they do not unless you specifically beforehand assign materia/files ownership as awell as assigning intellectual copyright. If they use your illustration for another piece of work or say a poster etc they are infingeing on your intellectual copyright and usage. It is not theirs do do with as they like, unless they have paid for that exclusive use. Now most agencies simply just give the stuff to the client - it is up to you. But it is like giving away the publishing rights to a song.
Do not release anything not even artwork files until you have agreement. You are under no legal obligation. Copyright infringement is a big deterrant for this type of thing.