Missing inline images in HTML mail

Inline images were not being rendered in HTML emails and I could find no code implementing function MsgHTML() of smtp.phpmailer.inc, so I altered function mail() of smtp.mail.inc to replace this:

case 'text/html':
  // The message includes only an HTML part.
  $mailer->IsHTML(TRUE);
  $content_type = 'text/html';
  break;

with this:

case 'text/html':
  // The message includes only an HTML part.
  $mailer->IsHTML(TRUE);
  $content_type = 'text/html';
  $mailer->MsgHTML($body);
  $body = $mailer->Body;
  break;

Am I doing the right thing in the right place (just 'cos it works doesn't mean that it's the correct answer)? Let me know and I'll roll proper patches for D7 beta2 and dev.