curl --request POST \
  --url https://sandbox.api.fin.com/v1/beneficiaries/{beneficiary_id}/documents \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: multipart/form-data' \
  --form doc1='@example-file'

import fs from 'fs';

const formData = new FormData();
formData.append('doc1', await new Response(fs.createReadStream('example-file')).blob());

const url = 'https://sandbox.api.fin.com/v1/beneficiaries/{beneficiary_id}/documents';
const options = {method: 'POST', headers: {Authorization: 'Bearer <token>'}, body: formData};

fetch(url, options)
  .then(res => res.json())
  .then(json => console.log(json))
  .catch(err => console.error(err));

HttpResponse<String> response = Unirest.post("https://sandbox.api.fin.com/v1/beneficiaries/{beneficiary_id}/documents")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "multipart/form-data")
  .body("-----011000010111000001101001\r\nContent-Disposition: form-data; name=\"doc1\"; filename=\"example-file\"\r\nContent-Type: application/octet-stream\r\n\r\n<string>\r\n-----011000010111000001101001--")
  .asString();

import requests

url = "https://sandbox.api.fin.com/v1/beneficiaries/{beneficiary_id}/documents"

files = { "doc1": ("example-file", open("example-file", "rb")) }
headers = {"Authorization": "Bearer <token>"}

response = requests.post(url, files=files, headers=headers)

print(response.text)

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://sandbox.api.fin.com/v1/beneficiaries/{beneficiary_id}/documents"

	payload := strings.NewReader("-----011000010111000001101001\r\nContent-Disposition: form-data; name=\"doc1\"; filename=\"example-file\"\r\nContent-Type: application/octet-stream\r\n\r\n<string>\r\n-----011000010111000001101001--")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Authorization", "Bearer <token>")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

{
  "data": {
    "files": [
      {
        "invoice1": "/wKbvfH5E_Invoice.pdf"
      },
      {
        "invoice2": "/XUOdWacK_Invoice.jpg"
      }
    ]
  }
}