Create Maintenance work order with attachments via rest api

I am developing an extension solution which integrates with oracle fusion SCM Maintenance module, one of the extensions functionalities is to post work orders on fusion with its attachments via REST.

I managed to do so in 2 steps
1. create work order first.
2. and post attachments to the created work order

with these documents below

https://docs.oracle.com/en/cloud/saas/supply-chain-and-manufacturing/24b/fasrp/op-maintenanceworkorders-post.html

https://docs.oracle.com/en/cloud/saas/supply-chain-and-manufacturing/24b/fasrp/op-maintenanceworkorders-workorderid-child-attachments-post.html

it's working just fine,
but when I need to create them in one POST Request it returns an error

I tried to do so with this payload for example: