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HTTP send requests can include up to 10 attachments. Each attachment is sent as base64 content inside the JSON body.

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Limits

LimitValue
Attachments per email10
Request body25 MB
Base64 content counts towards the request body size, so keep files below the full request limit.

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Allowed files

Sendmux accepts common image, document, spreadsheet, presentation, text, calendar, and contact formats. Executables and archives are rejected. Binary files are checked against their claimed file type. Text-based files are checked by extension.

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Add an attachment

{
  "from": { "email": "hello@example.com" },
  "to": { "email": "user@example.com" },
  "subject": "Invoice attached",
  "html_body": "<p>Please find your invoice attached.</p>",
  "attachments": [
    {
      "filename": "invoice-2026-03.pdf",
      "content": "JVBERi0xLjQKMSAwIG9iago8PAovVHlwZSAvQ2F0YWxvZw...",
      "type": "application/pdf"
    }
  ]
}

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Fields

FieldRequiredNotes
filenameYesMust include an allowed extension.
contentYesBase64-encoded file content.
typeNoOptional MIME type override.

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What happens next

If an attachment is too large, unsupported, or does not match its file type, the request fails with validation_error or payload_too_large. Fix the file and retry.

Send by HTTP

Add attachments to single or batch send requests.

Sending errors

Handle validation and payload size errors.