Send and download email attachments with Nylas

Email is how most files still move between people: the signed contract, the PDF invoice, the logo embedded in a newsletter. If your app sends or processes mail, it has to handle attachments, and doing that against each provider means Gmail's attachment encoding, Microsoft Graph's, and raw MIME for IMAP. The Nylas Email API gives you one model for both directions: attach files to outbound messages with the same call you use to send, and pull files off inbound messages with a read-only Attachments API. This post covers both halves from two angles: the HTTP API for your backend, and the nylas CLI for the terminal. I work on the CLI, so the terminal commands below are the ones I reach for when I'm checking a file came through. Two APIs: one to attach, one to read There's a split worth understanding up front. You add attachments through the Messages or Drafts API, as part of sending or saving a message, and you read existing attachments through the dedicated Attachments API. The Attachments API is read-only: it downloads bytes and returns metadata, but it never adds files. That division keeps the model simple, since attaching is part of composing a message and reading is a separate concern. The size of what you're attaching decides how you encode it on the way out. Small files ride inline in the JSON request, larger ones move to a multipart request, and very large files use a separate upload step. On the way in, every attachment, regardless of how it was sent, is fetched the same way: by its attachment_id together with the message_id it belongs to. Get those two ideas straight and the rest is mechanical. Attach a small file inline with Base64 For files that keep the whole request under 3 MB, the simplest path is the application/json schema. You pass each attachment in an attachments array with its content_type , filename , and the file bytes as a Base64-encoded content string. The 3 MB ceiling covers the entire HTTP request, not just the file, so it's the right path for