[5.0.x] Fixed #35657 -- Made FileField handle db_default values.

Backport of 8deb6bb1fc from main.

django/db/models/fields/files.py

@@ -8,6 +8,7 @@ from django.core.files.images import ImageFile
 from django.core.files.storage import Storage, default_storage
 from django.core.files.utils import validate_file_name
 from django.db.models import signals
+from django.db.models.expressions import DatabaseDefault
 from django.db.models.fields import Field
 from django.db.models.query_utils import DeferredAttribute
 from django.db.models.utils import AltersData
@@ -192,6 +193,12 @@ class FileDescriptor(DeferredAttribute):
             attr = self.field.attr_class(instance, self.field, file)
             instance.__dict__[self.field.attname] = attr
 
+        # If this value is a DatabaseDefault, initialize the attribute class
+        # for this field with its db_default value.
+        elif isinstance(file, DatabaseDefault):
+            attr = self.field.attr_class(instance, self.field, self.field.db_default)
+            instance.__dict__[self.field.attname] = attr
+
         # Other types of files may be assigned as well, but they need to have
         # the FieldFile interface added to them. Thus, we wrap any other type of
         # File inside a FieldFile (well, the field's attr_class, which is