Backup Set/What/File types -> All default file type options (Pictures, Music, ...) are set to "Off".
Only "Other File Types" are "On".
"Other File Types" includes only two file types defined ".css" and ".js".
When I run this backup set, Norton adds all file types (any) found in folders which are included in backup. Why?
Other file types is there for you to choose specific file types you wish to back up. The .css and .js types are just a couple of types Norton populated. You can add your own file types by clicking on Add New, and then remove the ones you do not want/need by clicking on the file type and clicking on Remove.
Or you can turn off the Other file types, and just add your own folders and files to backup. From your image, click on the Add or Exclude Files or Folders link above the default file types.