application icon change

If you change the icon of an application by:

1. Use Get Info to copy icon "Icon_C" from some file or application
2. Use Get Info to paste the icon "Icon_C" to Application_A such that Application_A now shows icon "Icon_C" instead of its normal icon

An invisible file named "Icon" will be generated at the root level of the Application_A bundle. This file likely contains the "Custom" icon plus ???

Does anyone know the format of this file and/or how to open it? If you copy it to the Desktop and double click on it you get the message

Thanks