How can I attach multiple pdfs to drupal_mail() function?
You forgot to add headers to the params array.

I did something like :

$attachment = array(
      'filecontent' =&gt; $filepathname,
      'filename' =&gt; $namefile,
      'filemime' =&gt; 'application/pdf'
      );
//where $filepathname should contain the path to the file and $filename should contain the name of the file.

$params = array(
  'headers' =&gt; array('Content-Type' =&gt; 'text/html'),
  'key' =&gt; 'test',
  'to' =&gt; 'test@example.com',
  'from' =&gt; 'test@example.com',
  'subject' =&gt; 'Test email',
  'body' =&gt; 'test',
  'attachment' =&gt; $attachment
);

drupal_mail($module, $key, $to, $language, $params, $from, $send);


which worked for me.

In your case:


Check the value of $filemime variable.
Add headers to the code.