Regression in construct_instance for FileField's with defaults

[ryanbutterfield]

Django 1.10 has added a regression in construct_instance in relation to FileField's with a default value (or any field that uses form.files).

This was added in ​https://github.com/django/django/commit/325dd0befea3012c42eefa061f509fbdf1b6a8aa and a separate regression fixed in ​https://github.com/django/django/commit/db3eabfae5c590801a74e0b318663b6480145fb9

The new code only checks for form.add_prefix(f.name) not in form.data. FileField's have their values in form.files and not in form.data, so this causes FileField's to be skipped if f.has_default() is True.

The correct code should be along the lines of:

 if (f.has_default() and 
                  form.add_prefix(f.name) not in form.data and
                  form.add_prefix(f.name) not in form.files and
                  not getattr(form[f.name].field.widget, 'dont_use_model_field_default_for_empty_data', False)):

[Simon Charette]

Duplicate of #27186, should be fixed in Django 1.10.1 by ​this patch.