Thanks for answering.
I am not worry about file type.
What I want is: If there are more than one picture for cover art, so there can be for example a picture that has cover-art-type "front" and an other that has cover-art-type "back". I want to save the pictures as "front.png" and "back.png". There for I am searching for a variable like %cover-art-type% to put this in the input mask:
Options -> Option... -> Cover Art -> using the following file name for