Attachments on transactional Motor Blocks. A receipt or an invoice with its PDF is ordinary transactional mail, so transactional Motor Blocks now send attachments over SMTP, exactly like general_purpose ones. Before, they were refused with "Use a general_purpose motor block to send attachments". Your plan still decides whether attachments are included, how many per message and the largest file. Marketing blocks still do not send attachments.
The real size ceiling is 10 MB for the whole message. Files are base64-encoded for transport, so keep the total of all files under about 7 MB. This ceiling applies whatever your plan's per-file figure is.
Clear permanent errors. A message over 10 MB now gets 552 5.3.4 Message size exceeds limit (10MB), and a message with more than 100 recipients gets 452 4.5.3. Before, both returned a temporary 450, so a mail client kept retrying a message that could never be accepted. A message with no subject is now refused with 550 5.6.0.
New guide:Send attachments, with Node.js, Python and PHP examples. The HTTP API (POST /v1/send) still sends text and HTML only, with no attachments and no cc or bcc; the docs now say so plainly.