8 – How to send system mails in html format (with Swift Mailer)?

At Account settings page (/admin/config/people/accounts) I can edit system mails Drupal is sending (Welcome, Account activated, Account blocked and so on).

I also installed and configured Swift Mailer module. At Message settings page (admin/config/swiftmailer/messages) I selected HTML for message format.

Then I edit some message template and enter html content there like:

<!DOCTYPE html>
<html>
  <head>
    <title>Your account at (site:name) has been activated.</title>
  </head>
  <body>
(user:display-name),

Your account at (site:name) has been activated.

You may now log in by clicking this link or copying and pasting it into your browser:

(user:one-time-login-url)

This link can only be used once to log in and will lead you to a page where you can set your password.

After setting your password, you will be able to log in at (site:login-url) in the future using:

username: (user:account-name)
password: Your password

--  (site:name) team
  </body>
</html>

However in mail I receive I still got Content-Type: text/plain; and I see it as plain text.

At that Swift Mail Message settings page it says:

“You can set the default message format which should be applied to e-mail messages.”

But I don’t see that option anywhere? How to change this default message format (to “text/html”)?

An update:

I found out that if I uncheck “Respect provided e-mail format.” on SwiftMails Message settings page I do get text/html content type in mail header. However all the tags I enter in mail template are stripped out and new rows are automatically converted to paragraphs?
I expected that there will be “HTML email” text format at Text formats page (/admin/config/content/formats) where I can set filters, but there aren’t any.