When attaching a file to an email it is best to stick to the smallest size possible. (ReachMail allows the attachments of files 500K and smaller) Even better, upload the file to the Tools -> Content & Assets area of your account and then insert a link to file in the body of your message. Attaching a file to an email can slow the delivery process, or even cause the message to be rejected. Some mail servers do not accept emails with attachments. While attaching a file to an email is generally not an issue with person to person emails, it is much different when it comes to mass marketing promotions.
Once you have uploaded your file to the Contents & Assets location in your account (we'll host the file for you for the lifetime of your account) you can insert a link to that file using the following toolbar button from the email editor: