Generally it is not a good idea to add attachments to mass e-mails in CiviCRM. Delivering each attachment consumes your bandwidth. If you have 1,000 contacts and you deliver each one a 1 MB attachment then you will have used 1 GB of bandwidth.
CiviCRM has controls that allow you to limit attachments in e-mail messages.
Navigate to Administer | System Settings | Undelete, Logging and ReCAPTCHA and set the e-mail attachment limits.
You can set the number of files and the maximum file size.
A less bandwidth-intensive method is to use the Link tool in the rich text editor to upload large files to your server and simply put links to them in your mail.