How can I suggest a template file specific for a content type?
You can do this with a theme suggestions alter hook:

/**
 * Implements THEME_theme_suggestions_html_alter().
 */
function MYTHEME_theme_suggestions_html_alter(&amp;$suggestions, $vars, $hook) {

  // Check to see if you are on a node.
  if ($node = \Drupal::routeMatch()-&gt;getParameter('node')) {

    // Get the node type.
    $type = $node-&gt;getType();

    // Create the theme hook suggestions.
    $suggestions[] = 'html__' . $type;
    $suggestions[] = 'html__node_' . $type;
  }
}


Of course, change "MYTHEME to your theme name. I tested this on a blog content type for example and the new template suggestions printed out after creating this hook and clearing cache. 

&lt;!-- THEME HOOK: 'html' --&gt;
&lt;!-- FILE NAME SUGGESTIONS:
   * html--node-blog.html.twig
   * html--blog.html.twig
   * html--node--15.html.twig
   * html--node--%.html.twig
   * html--node.html.twig
   x html.html.twig
--&gt;


The new theme hooks here are:

  * html--node-blog.html.twig
  * html--blog.html.twig


The content type name will change accordingly in the naming suggestion. 

To test, I copied html.html.twig and named as html--node-blog.html.twig and it worked great.