Images not uploaded using picture field

When using Picture filed type on Database Activity, images are not uploaded.

To reproduce:

1. Create a DB activity
2. Add only a Picture Field
3. Create default templates
4. Now try adding a entry: select a image file on your computer, enter the alternate text and click either "Save and view" or "Save and add another".
5. In both cases Moodle displays the message "You did not fill out any fields!"