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The send API accepts attachments as base64-encoded JSON objects. Signal decodes them before enforcing the configured total attachment limit.

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Attachment fields

For an inline image, set disposition to inline, provide a contentId, and reference the same value from HTML with a cid: URL. A content ID alone does not make a regular attachment render inline.

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Size accounting

Signal applies its attachment ceiling to decoded bytes, not the larger base64 text length. The exact configured ceiling can vary by deployment or entitlement; use the validation response and current product contract instead of assuming a copied number. The complete MIME message can contain at most 500 parts, including HTML and plain-text body parts as well as attachments. A request with many small files can therefore fail the part count even when decoded bytes remain below the size ceiling.

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Common validation failures

  • The base64 string is truncated or contains a data-URL prefix.
  • The filename includes /, \, or control characters.
  • contentType is blank.
  • The decoded total exceeds the configured limit.
  • Body parts plus attachments exceed 500.
  • HTML references a cid that does not match any attachment contentId.

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Diagnostic sequence

  1. Decode each value locally and compare byte counts with the source files.
  2. Send the smallest failing attachment by itself.
  3. Remove any data:...;base64, prefix; send only the base64 payload.
  4. Confirm filename, media type, disposition, and content ID.
  5. Inspect the 400 problem response for the failing field or configured boundary.
  6. Use test mode to confirm API admission, then use one controlled real message to verify mailbox rendering.
Base64 increases the JSON request size but does not change the decoded-file limit. Do not compare only the HTTP body length when diagnosing an attachment rejection.