> ## Documentation Index
> Fetch the complete documentation index at: https://docs.apollodeploy.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Attachment requirements and limits

> Build valid attachment payloads and diagnose size, encoding, and MIME-part failures

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

## Attachment fields

| Field         | Requirement                                               |
| ------------- | --------------------------------------------------------- |
| `filename`    | A file name without path separators or control characters |
| `content`     | Valid base64 for the complete file bytes                  |
| `contentType` | A non-empty MIME media type                               |
| `disposition` | `attachment` by default, or `inline`                      |
| `contentId`   | Optional identifier for an inline body reference          |

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.

## 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.

## 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`.

## 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.

<Warning>
  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.
</Warning>