The list header keyword says your list will not accept files. But of
course there still exists a field in the user subscription which says
whether this particular user wants to get files. There is no reason for
LISTSERV to change these fields, with "Files= NO" you cannot send files
anyway, and you'd be sorry it had changed them if you decided to
re-enable files 2 days later after numerous complaints. The default value
of this field for new users is controlled by "Default-Options=" like all
other options.
Eric