To attach a file to an email, open a contact, click "Send Email", then choose "Attach file(s)".
Then select the files you would like to add (you can select multiple) and click "Open".
You can use the trash can next to the uploaded attachment to remove individual files if needed.
Batch emails & attachments
If you want to send a file in a batch email, it is best to upload the file to a hosting service such as Dropbox or Google Drive and include a link to the file in the email. As is normal with email marketing, files cannot be attached to batch emails.