Fixed #29459 -- Initialized form data/files with empty MultiValueDicts.

django/forms/forms.py

@@ -11,6 +11,7 @@ from django.forms.fields import Field, FileField
 # pretty_name is imported for backwards compatibility in Django 1.9
 from django.forms.utils import ErrorDict, ErrorList, pretty_name  # NOQA
 from django.forms.widgets import Media, MediaDefiningClass
+from django.utils.datastructures import MultiValueDict
 from django.utils.functional import cached_property
 from django.utils.html import conditional_escape, html_safe
 from django.utils.safestring import mark_safe
@@ -69,8 +70,8 @@ class BaseForm:
                  initial=None, error_class=ErrorList, label_suffix=None,
                  empty_permitted=False, field_order=None, use_required_attribute=None, renderer=None):
         self.is_bound = data is not None or files is not None
-        self.data = {} if data is None else data
-        self.files = {} if files is None else files
+        self.data = MultiValueDict() if data is None else data
+        self.files = MultiValueDict() if files is None else files
         self.auto_id = auto_id
         if prefix is not None:
             self.prefix = prefix