Hello , File related list & attchment  related list is available in all the objects . But in Email message object only attachment related list is available .
My requirement in email Service is that I need to process attchment of incoming email as file .
So now I am able to attach email attchment to case object as a file using "Content Document" Object  and attchment is visible as file related list and attchment related list as well.
  BUt when it comes to attch incoming attchment to Email message , then Its not working .
Email message do not have file related list only attchment related list .
Can you please suggest anything ??