multipart/alternative vs. multipart/mixed in Content-Type header

I noticed that when I send an attachment to a Yahoo! mail account, attachments weren't being received.

I checked the headers and saw that I was sending Content-type: multipart/alternative when Yahoo! demanded Content-type: multipart/mixed. Other mail providers, e.g. Gmail, don't seem to care (h/t http://stackoverflow.com/questions/10873269/correct-email-format-for-ema...).

I'm not 100% sure that this is a Mail MIME issue, but it is the only module I'm using that touches on the Content-type headers at all. The parseDecoded function in mailmime.inc doesn't seem to have the capability to add a 'multipart/mixed' header to the MIME object.

My code for sending the attachments goes:

function my_module_send_email($to, $subject, $message, $attachments = array()) {
  $params = array(
    'subject' => $subject,
    'message' => $message,
  );

  $message = drupal_mail('my_module', $key, $to, 'und', $params, NULL, FALSE);

  $mime = &$message['MailMIME'];

  if (sizeof($attachments)) {
    foreach ($attachments as $file) {
      $attachment = array();
      $attachment['content'] = file_get_contents(drupal_realpath('private://' . $file->filename));
      $attachment['filename'] = $file->filename;
      $attachment['filemime'] = $file->filemime;
      $attachment['encoding'] = 'base64';
      $attachment['disposition'] = 'attachment';
      $attachment['charset'] = '';
      $attachment['language'] = '';

      $mime->addAttachment(
        $attachment['content'],
        $attachment['filemime'],
        $attachment['filename'],
        FALSE,
        $attachment['encoding'],
        $attachment['disposition'],
        $attachment['charset'],
        $attachment['language']
      );
    }
  }

  if (method_exists($mime, 'get')) {
    $message['body'] = $mime->get();
  }
  $system = drupal_mail_system(NULL, NULL);
  $message['result'] = $system->mail($message);

  return $message['result'];

To get the correct header, I added some lines right before the call to $system->mail:

  if (sizeof($attachments)) {
    $message['headers']['Content-Type'] = str_replace('alternative', 'mixed', $message['headers']['Content-Type']);
  }

So possibly this is a bug, and possibly I am just sending mail in a funky way. Any feedback would be helpful.