Set GroupOwner in Proftpd
Due to an error in proftpd, you might need an virtual group.
/etc/proftpd/proftpd.conf:
GroupOwner www-data
Umask 002 003
AllowOverwrite on
AuthGroupFile /etc/proftpd/ftpd.group
/etc/proftpd/ftpd.group
www-data:x:33:testuser
Same ID and name as the local user.
