Implements hook_field_delete().
function file_field_delete($entity_type, $entity, $field, $instance, $langcode, &$items) {
list($id, $vid, $bundle) = entity_extract_ids($entity_type, $entity);
// Delete all file usages within this entity.
foreach ($items as $delta => $item) {
file_field_delete_file($item, $field, $entity_type, $id, 0);
}
}