This the code is from the "DrupalCon Baltimore 2017: Demystifying Rendered Content in Drupal 8 Twig Files" presentation by Amy Vaillancourt-Sals slide 30 "Create a custom file name suggestion". It adds Twig templates for block bundles.

themename.theme

/**
 * This code is from the "DrupalCon Baltimore 2017: Demystifying Rendered Content in Drupal 8 Twig Files" presentation by Amy Vaillancourt-Sals slide 30 "Create a custom file name suggestion".
 * Video: https://www.youtube.com/watch?v=qoeRpRkGwmk
 * Slides: https://drive.google.com/file/d/0B8g8dQw8nc9eV3U4SjZKNWNEWGM/view?usp=sharing
 * Drupalcon Baltimore Session: https://events.drupal.org/baltimore2017/sessions/demystifying-rendered-content-drupal-8-twig-files
 * This code adds Twig templates for block bundles. Replace "themename" below with your theme's machine name.
 * Implements hook_theme_suggestions_HOOK_alter() for templates.
 * @param array $suggestions
 * @param array $variables
 */
function themename_theme_suggestions_block_alter(array &$suggestions, array $variables) {
    // Block suggestions for custom block bundles.
    if  (isset($variables['elements']['content']['#block_content'])) {
       array_splice($suggestions, 1,0, 'block__type__' . $variables['elements']['content']['#block_content']->bundle());
    }
}