Android Custom Activity

According to the documentation under the Android plugin section, when you build an activity extending UnityActivity you have to compress the resulting .class files into a .jar file and place it under Assets->Plugins->Android. I have my Jar file stored over there, with the classes.jar name, nevertheless when I run the game it does using UnityActivity and it can't even find muy class by JNI. Is there a special name for the file, or an extra setup I've got to do?