I don't see anything obviously wrong in your sample, but after comparing your implementation with mine from a recent project, I see a couple of differences:

1. You mention using the mimemail module, but are you also using the mailsystem module?  The simplest explanation would be that mimemail is not getting invoked at all.
2. If I recall correctly, some fields you can set in the "message" array upstream can get unset before hook_mail is called for you.  So you may need to put these items into your "params" array and hammer them in as part of your hook_mail implementation.  My code looks like this:

Sending the mail:

  $mail_params = array(
    'subject' => $settings['subject'],
    'message' => $settings['message'],
    'headers' => array(
      'Content-Type' => 'text/html; charset=UTF-8'
    ),
    'attachments' => $attachments,
    'voter' => $credit_app,
  );
  $mail_params['plain'] = FALSE;
  $mail_params['plaintext'] = $mail_params['message'];
  if (!empty($settings['bcc'])) {
    $mail_params['headers']['Bcc'] = $settings['bcc'];
  }
  
  $message = drupal_mail('mymodule', 'send-test', $address, $language, $mail_params, $from);

In my hook_mail implementation:

  switch ($key) {
    case 'send-test':
      $message['subject'] = $params['subject'];
      $message['body'][] = $params['message'];
      $message['headers'] = array_merge($message['headers'], $params['headers']);
      break;
  }  

Note also that I'm setting some parameters that you are not setting ('plain', 'plaintext');  I believe I added those after debugging the problem you're having here.

Hope this helps,

Rob

Rob Thorne
Torenware Networks



On Dec 29, 2013, at 12:50 PM, Muzaffer Tolga Ozses <tolga@ozses.net> wrote:

Hi,

I keep developing my module, and inside it I am sending e-mail. I am also trying to attach a file, to no avail. I have installed mimemail, I have changed mime type in my code, I have dpm'd everything. dpm gives me correct values, and the subject and body of the e-mail come through, but I couldn't get file attachment to work at all. What else can I do?

Regards,