string $body - The body of the mime part if any. Default is an empty string.
array $params - An associative array of parameters:
$params["content_type"] - The content type for this part ie. multipart/mixed
$params["encoding"] - The encoding to use ie. 7bit, 8bit, base64 or quoted-printable
$params["cid"] - content ID to apply
$params["disposition"] - Content disposition inline or attachment
$params["dfilename"] - Optional filename parameter for content disposition
$params["description"] - Content description
$params["charset"] - Character set to use