I think I have solved the problem.
While I dug in the library sources I found this code in
formencode/schema.py:
# are iterators (list, tuple, set, etc) allowed?
if self._value_is_iterator(value) and not
getattr(validator, 'accept_iterator', False):
errors[name] =
Invalid(self.message('singleValueExpected', state),
value_dict, state)
so I deduced that the validator FileField must have the parameter
accept_iterator=True
as in:
w.FileField(name='modello',
validator=v.FieldStorageUploadConverter(accept_iterator=True))
and now it works.
j