Attachments that are not Images or Content (data) get left out.

In working with the latest DEV version. The function _mimemail_file has been changed/updated to only pass through images when sending an absolute path to the $url parameter.

In file mimemail.inc on line 156.

I have updated the code as such:

if ($url) {
    // The file exists on the server as-is. Allows for non-web-accessible files.
    if (@is_file($url)) {
      $file = $url;
    }
    else {
      $image = preg_match('!\.(png|gif|jpg|jpeg)$!i', $url);
      $linkonly = variable_get('mimemail_linkonly', 0);
      // The file exists on the server as-is. Allows for non-web-accessible files.
      if (@is_file($url) && $image && !$linkonly) {
        $file = $url;
      }
      else {
        $url = _mimemail_url($url, 'TRUE');
        // The $url is absolute, we're done here.
        $scheme = file_uri_scheme($url);
        if ($scheme == 'http' || $scheme == 'https' || preg_match('!mailto:!', $url)) {
          return $url;
        }
        // The $url is a non-local URI that needs to be converted to a URL.
        else {
          $file = (drupal_realpath($url)) ? drupal_realpath($url) : file_create_url($url);
        }
      }
    }
  }

It was:

if ($url) {
    $image = preg_match('!\.(png|gif|jpg|jpeg)$!i', $url);
    $linkonly = variable_get('mimemail_linkonly', 0);
    // The file exists on the server as-is. Allows for non-web-accessible files.
    if (@is_file($url) && $image && !$linkonly) {
      $file = $url;
    }
    else {
      $url = _mimemail_url($url, 'TRUE');
      // The $url is absolute, we're done here.
      $scheme = file_uri_scheme($url);
      if ($scheme == 'http' || $scheme == 'https' || preg_match('!mailto:!', $url)) {
        return $url;
      }
      // The $url is a non-local URI that needs to be converted to a URL.
      else {
        $file = (drupal_realpath($url)) ? drupal_realpath($url) : file_create_url($url);
      }
    }
  }

I can supply a patch if this is the way you want it to be handled. Not sure why you changed it originally so I don't want to assume anything. I am sending out PDF attachements so this hindered my progress.

But instead of changing the code as in above, I have altered my rule to not send the absolute path and instead send the uri. It will pick up my PDF that way. I just think there needs to be some standard way of sending files to this function. Seems overly complicated to me, but again I don't want to assume on the why part.

So my adding attachements through rules went from:
echo drupal_realpath(file_load($file['fid'])->uri);
to:
echo file_load($file['fid'])->uri;