extra_isos: Mention all extra files in the manifest
    
    When container-images are downloaded, they would be skipped from the
    extra_files.json manifest. This patch fixes that by enumerating all
    files rather than relying on the getter to return a list.
    
    JIRA: RHELCMP-14406
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>