code: function addAttachmentFromFile($mailingId, $filename, $contentType, $attachmentFileName = null) {
          $handle = fopen($filename, "rb");
          if (FALSE === $filename) {
              throw new MaileonAPIException("Cannot read file " . $filename . ".");
          }
          $contents = '';
          while (!feof($handle)) {
              $contents .= fread($handle, 8192);
          }
          fclose($handle);
          if ($attachmentFileName === null) {
              $attachmentFileName = basename($filename);
          }
          
          return $this->addAttachment($mailingId, $attachmentFileName, $contentType, $contents);
      }
docs: Read a binary file from the file system and adds it as an attachment to this transaction.
  
  @param type $mailingId
  @param type $filename
  @param type $contentType
  @param type $attachmentFileName Name of the file in the attachments
  
  @return com_maileon_api_MaileonAPIResult
  @throws com_maileon_api_MaileonAPIException