Nextjs proxy multipart form data with fetch

I am trying to proxy all api calls to a service with nextjs. So far this is what I achieved

import { NextApiRequest, NextApiResponse } from "next"
import { getSession } from 'next-auth/client'
import Cookies from 'cookies'
import { apiURL } from 'apps/dashboard/config'

const theProxy = async (req: NextApiRequest, res: NextApiResponse) => {
    const session = await getSession({ req })

    const { method, query, headers, body } = req

    const cookies = new Cookies(req, res)
    let {path, ...params} = query
    path = (path as string[]).join("/")
    const searchParams = toQueryString(params)

    // const authorization = `Bearer ${session.user['remoteToken']}`
    let payload = null
    if(headers['content-type'] && headers['content-type'].indexOf("multipart/form-data") > -1){
        payload = body
    } else {
        payload = JSON.stringify(body)
    }

    const options = {
        method,
        body: method != "GET" && method != "HEAD" ? payload : undefined,
        headers: {
            'Content-Type': headers['content-type'] || "application/json",
            // 'Authorization':  authorization,
            ...(session ? {'cookie':  session?.user['cookies']} : {}),
            sessionID: cookies.get("sessionID")
        }
    }

    if(headers['content-length']){
        options.headers["Content-Length"] = headers['content-length']
    }

    const response = await fetch(`${apiURL}/${path}?${searchParams}`, options)

    try {
        const respBody = await response.json()
        res.status(response.status).json(respBody)
    } catch (error) {
        res.status(response.status).end(response.statusText)
    }
}

This is working except file upload (multipart). And it looks like uploading but not working properly.

Any suggestions?