What can not be used to fill a fire extinguisher?

You can fill a fire extinguisher with any substance you like. Whether it will work as a fire extinguisher afterwards is a different matter. The old pump-style water fire extinguishers in college dormitories have occasionally been found to contain beer (which at least still works for the intended purpose) or gasoline (which is not so effective), which is one reason you don't see that type so much anymore.