Official comment
Email attachments are no longer supported in any bulk email either via Rules, Sequences or Group actions. Delivery of bulk email is reduced significantly when there is an attachment. You can upload the file into the File Manager and provide the link to that file. Your Contact can then click the link to download the file to their computer. The download link does not trigger spam filters and does not affect deliverability. To access the File Manager, click your email address at the top right, then "Administration". The File Manager link is the first one on the top left column of links. After uploading the file, click on the Name of the file in the File Manager to expose the download link.
The only other option for attaching a file to an email is when you send a one-off email to a single Contact. You can attach a file from the Contact Record's Filebox in that case. This allows you to store personalized documents such as contracts, etc. in the Contact Record and attach them when sending an email manually. You cannot automate storing documents in the File Box, however, and cannot attach the file when sending email through a Group Action or Sequence.