Clarify message attachment rendering in webhooks (#1499)

* Clarify message attachment rendering in webhooks

Added a section explaining message attachment rendering requirements for webhooks.

* Update site/content/integrate/webhooks/incoming/_index.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: svelle

site/content/integrate/webhooks/incoming/_index.md

@@ -197,5 +197,8 @@ Some common error messages include:
 
 If your integration prints the JSON payload data instead of rendering the generated message, make sure your integration is returning the `application/json` content-type.
 
+#### Why aren't my message attachments rendering?
+Make sure `attachments` is a **top-level field** in your webhook JSON payload, not nested inside `props`. While the REST API uses `props.attachments`, incoming webhooks expect `attachments` at the top level.
+
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