Call a Upload File Web API within another Web API

Hi

I need to start different process models in a WEB API based on the path segment. One of the process models would upload a document from an external system.
A web api is created using "upload document" template. This web api is called in an integration where the document is passed as a part of "multipart/form-data" and other parameters are passed as query parameters and headers.

I tried calling this integration in parent web api, I get Internal Server Error.

If I try calling upload document code in the parent web api also I get the same error.

How can we resolve this?

Appian version : 21.1