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

Backport of 8deb6bb1fc from main.

tests/file_storage/models.py

@@ -72,6 +72,9 @@ class Storage(models.Model):
     default = models.FileField(
         storage=temp_storage, upload_to="tests", default="tests/default.txt"
     )
+    db_default = models.FileField(
+        storage=temp_storage, upload_to="tests", db_default="tests/db_default.txt"
+    )
     empty = models.FileField(storage=temp_storage)
     limited_length = models.FileField(
         storage=temp_storage, upload_to="tests", max_length=20

tests/file_storage/tests.py

@@ -944,6 +944,20 @@ class FileFieldStorageTests(TestCase):
         self.assertEqual(obj.default.read(), b"default content")
         obj.default.close()
 
+    def test_filefield_db_default(self):
+        temp_storage.save("tests/db_default.txt", ContentFile("default content"))
+        obj = Storage.objects.create()
+        self.assertEqual(obj.db_default.name, "tests/db_default.txt")
+        self.assertEqual(obj.db_default.read(), b"default content")
+        obj.db_default.close()
+
+        # File is not deleted, even if there are no more objects using it.
+        obj.delete()
+        s = Storage()
+        self.assertEqual(s.db_default.name, "tests/db_default.txt")
+        self.assertEqual(s.db_default.read(), b"default content")
+        s.db_default.close()
+
     def test_empty_upload_to(self):
         # upload_to can be empty, meaning it does not use subdirectory.
         obj = Storage()