Fix FileFields in deeply nested inlines

Django 2.1+ implemented 'has_file_field', this value no longer is
hardcoded to 'True', but depends on 'is_multipart()'.

Django relies on 'has_file_field' to set form's enctype to multipart.
With false negatives in 'has_file_field', FileField changes are not
saved.

This commit adds support for nested_formsets to 'is_multipart()'.

nested_admin/formsets.py

@@ -24,17 +24,69 @@ def mutable_querydict(qd):
         qd._mutable = orig_mutable
 
 
+PATCH_FORM_IS_MULTIPART = ((2, 1) < django.VERSION < (3, 0))
+
+
+class FixDjango2MultipartFormMixin(object):
+    def is_multipart(self, check_formset=True):
+        """
+        Overridden is_multipart for Django 2.1 and 2.2 that returns the
+        formset's is_multipart by default.
+
+        Parameters
+        ----------
+        check_formset : bool (default=True)
+            If ``False``, returns the form's original is_multipart value.
+            Exists to prevent infinite recursion in the formset's is_multipart
+            lookup.
+        """
+        parent_formset = getattr(self, 'parent_formset', None)
+        if check_formset and parent_formset:
+            return parent_formset.is_multipart()
+        else:
+            return super(FixDjango2MultipartFormMixin, self).is_multipart()
+
+
 class NestedInlineFormSetMixin(object):
 
     is_nested = False
 
+    def __init__(self, *args, **kwargs):
+        super(NestedInlineFormSetMixin, self).__init__(*args, **kwargs)
+        if PATCH_FORM_IS_MULTIPART:
+            self.form = type(
+                self.form.__name__, (FixDjango2MultipartFormMixin, self.form), {
+                    'parent_formset': self,
+                })
+
     def _construct_form(self, i, **kwargs):
         defaults = {}
         if '-empty-' in self.prefix:
             defaults['empty_permitted'] = True
         defaults.update(kwargs)
         return super(NestedInlineFormSetMixin, self)._construct_form(i, **defaults)
 
+    def is_multipart(self):
+        if not PATCH_FORM_IS_MULTIPART:
+            if super(NestedInlineFormSetMixin, self).is_multipart():
+                return True
+        else:
+            forms = [f for f in self]
+            if not forms:
+                if hasattr(type(self), 'empty_forms'):
+                    forms = self.empty_forms  # django-polymorphic compat
+                else:
+                    forms = [self.empty_form]
+            for form in forms:
+                if form.is_multipart(check_formset=False):
+                    return True
+
+        for nested_formset in getattr(self, 'nested_formsets', []):
+            if nested_formset.is_multipart():
+                return True
+
+        return False
+
     def save(self, commit=True):
         """
         Saves model instances for every form, adding and changing instances

nested_admin/tests/two_deep/models.py

@@ -62,6 +62,7 @@ class Meta:
 
 class StackedItem(ItemAbstract):
     section = ForeignKey(StackedSection, related_name='item_set', on_delete=CASCADE)
+    upload = models.FileField(blank=True, null=True, upload_to='foo')
 
     class Meta:
         ordering = ('section', 'position')

nested_admin/tests/two_deep/tests.py

@@ -1,3 +1,5 @@
+import os
+import tempfile
 import time
 from unittest import skipIf, SkipTest
 
@@ -842,6 +844,29 @@ def test_add_item_inline_label_update(self):
         inline_label = self.get_item([0, 1]).find_element_by_class_name('inline_label')
         self.assertEqual(inline_label.text, '#2')
 
+    def test_upload_file(self):
+        group = self.root_model.objects.create(slug='group')
+
+        self.load_admin(group)
+
+        self.add_inline(slug="a")
+        self.add_inline(indexes=[0], name='A 0')
+
+        fd, path = tempfile.mkstemp()
+        try:
+            with os.fdopen(fd, 'w') as tmp:
+                tmp.write('Test file. Used as a payload for testing file uploads.')
+            with self.clickable_xpath('//input[@name="section_set-0-item_set-0-upload"]') as el:
+                el.send_keys(path)
+            self.save_form()
+        finally:
+            os.remove(path)
+
+        item_a_0 = self.item_cls.objects.get(name='A 0')
+        upload_name = 'foo/' + os.path.basename(path)
+
+        self.assertEqual(item_a_0.upload.name, upload_name, 'File upload failed')
+
 
 class TestTabularInlineAdmin(InlineAdminTestCaseMixin, BaseNestedAdminTestCase):