fix pb with bytes field processing: currently when an existing file is edited and no new file is specified, it finds no value in req.form and so try to detach the current file, which is wrong. In that case, nothing should be done. So introduce a new UnmodifiedField exception that may be raised in field's process_form_value method (catched in field.process_posted).

"""pre 3.6 bw compat"""
# pylint: disable-msg=W0614,W0401
from warnings import warn
warn('moved to cubicweb.mttransforms', DeprecationWarning, stacklevel=2)
from cubicweb.mttransforms import *