code: function addAttachment($mailingId, $filename, $contentType, $contents) {
          $queryParameters = array( 'filename' => $filename );
          return $this->post("mailings/${mailingId}/attachments", $contents, $queryParameters, null, null, $contentType, strlen($contents));
      }
docs: Adds an attachment to the mailing with the provided id.
  
  @param int $mailingId The mailing id
  @param string $filename Filename of the attachment to be displayed in sent emails. It is recommended to keep the filename short and to use an extension corresponding to the mime type of the attachment.
  @param string $contentType The mime type of the attachment
  @param string $contents The file content
  
  @return com_maileon_api_MaileonAPIResult