Are you using Gmail from webmail(by using in www.gmail.com)
Please go to www.gmail.com and login to your account. In inbox, click and open the message that you want to forward. After messages opens, you will see forward button, click it, between subject box and message box you will see attachment part, you will see attachments, check the attachments that you want to forward, click add button if you want to add more.
Please write if this solution does not work for you.