addAttachment() causing Strict-standard error

Change 1: Function signature - added 5th parameter for content `disposition` because its absense caused a Strict-Standard error to be thrown *Strict Standards: Declaration of JMail::addAttachment() should be compatible with PHPMailer::addAttachment($path, $name = '', $encoding = 'base64', $type = '', $disposition = 'attachment') in /libraries/joomla/mail/mail.php on line 22*

Change 2: The function parameters `encoding, type, disposition` are always of type `string`. I changed the type note to reflect this.