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The short answer: attachments go in the attachments field of the request itself, in base64, each with the file name and content. The total is up to 5 MB per e-mail and executable files are refused. For big files, send a link instead of an attachment.
How an attachment is sent
| Field of each attachment | What it takes |
| filename | The name the customer sees, such as invoice-FT2026-214.pdf |
| content | The file content in base64 |
| content_type | The type, such as application/pdf |
"attachments": [{"filename": "invoice.pdf",
"content": "JVBERi0xLjQK...",
"content_type": "application/pdf"}]
In PHP, for example, the content comes from base64_encode(file_get_contents($path)). Remember that base64 makes files bigger: a PDF weighs more inside the request than on disk.
When an attachment is refused
| Reason | What it means | What to do |
| invalid_attachment | The name is missing or the content is not valid base64 | Check the filename and the encoding |
| attachment_blocked | This file type cannot be attached (executables) | Send another format, such as PDF |
| attachments_too_large | The attachments are over the size limit | Compress them or send a link |
| too_many_attachments | Too many attachments in one e-mail | Put them in one file or split the send |
These reasons come in details.reason, in a 422 answer. The others are in Email Center API errors.
Attachment or link
| Use an attachment | Use a link |
| A light invoice, receipt or ticket in PDF | Catalogues, videos, folders of documents |
| A document the customer must keep | Something that changes after it is sent |
| When the customer may not have access to your client area | Files close to the limit, or for many recipients |
| Every recipient gets the whole attachment. A heavy file to 50 recipients is a lot of mail. And some servers refuse large messages: see attachment too large. |
| Say in the text what is attached, with the name and amount: "Attached is invoice FT 2026/214, for 1,250.00". An attachment with no explanation looks like phishing. |
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Want to send your PDF invoices through the API? See the API page. See the API page |
