Can a Webhook's Body combine a JSON part with a File Attachment part (multipart/form-data)?

Hello all,

I’m calling an internal mail API from a Foundry function via an External Systems webhook. The API requires multipart/form-data with two parts: a JSON part (subject, sender, recipients, etc.) and a binary part (the file).

My webhook has an attachment-type input parameter, and the function passes it through via .call({…}). The email sends fine, but the attachment never arrives.

In the webhook’s Request Configuration → Calls → Body, selecting “File Attachment” as the Body type only lets me bind that single Attachment parameter as the whole body — there’s no way to also send the other fields as a JSON part in the same request.

Question: Does the Webhook resource support a true multipart body with multiple parts (JSON + binary) in one request? If not, what’s the recommended way to call an API that requires this?

Thank you.