[bfss] fix file update to ensure file's content is available on the fs asap... and not only at commit time. So it's consistent with entity creation behaviour. The new file is created at assignement time and removed if the commit is rollbacked.

# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s views/forms/actions/components for web ui"""