Attachments is a controversial topic when sending cold emails to your contacts. Generally speaking, adding attachments have a high chance of sending your email into the spam box and therefore leading to a lower open rate. This is why you currently cannot attach a file directly into your email.
You can add attachments through links though. Ideally, you'd want your file hosted on your website but you can also use Google Drive and Dropbox to do this too. The added extra benefit is that you can also track it when someone opens that file too in your contact list.