curl --request POST \
  --url https://api.maesn.dev/accounting/bookingProposals/{bookingProposalId}/attachments \
  --header 'Content-Type: multipart/form-data' \
  --header 'X-ACCOUNT-KEY: <x-account-key>' \
  --header 'X-API-KEY: <x-api-key>' \
  --form file=@example-file

import requests

url = "https://api.maesn.dev/accounting/bookingProposals/{bookingProposalId}/attachments"

files = { "file": ("example-file", open("example-file", "rb")) }
headers = {
    "X-API-KEY": "<x-api-key>",
    "X-ACCOUNT-KEY": "<x-account-key>"
}

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

print(response.text)

const form = new FormData();
form.append('file', '<string>');

const options = {
  method: 'POST',
  headers: {'X-API-KEY': '<x-api-key>', 'X-ACCOUNT-KEY': '<x-account-key>'}
};

options.body = form;

fetch('https://api.maesn.dev/accounting/bookingProposals/{bookingProposalId}/attachments', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.maesn.dev/accounting/bookingProposals/{bookingProposalId}/attachments",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => "-----011000010111000001101001\r\nContent-Disposition: form-data; name=\"file\"; filename=\"example-file\"\r\nContent-Type: application/octet-stream\r\n\r\n<string>\r\n-----011000010111000001101001--",
  CURLOPT_HTTPHEADER => [
    "Content-Type: multipart/form-data",
    "X-ACCOUNT-KEY: <x-account-key>",
    "X-API-KEY: <x-api-key>"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

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

func main() {

	url := "https://api.maesn.dev/accounting/bookingProposals/{bookingProposalId}/attachments"

	payload := strings.NewReader("-----011000010111000001101001\r\nContent-Disposition: form-data; name=\"file\"; 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("X-API-KEY", "<x-api-key>")
	req.Header.Add("X-ACCOUNT-KEY", "<x-account-key>")

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

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

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://api.maesn.dev/accounting/bookingProposals/{bookingProposalId}/attachments")
  .header("X-API-KEY", "<x-api-key>")
  .header("X-ACCOUNT-KEY", "<x-account-key>")
  .body("-----011000010111000001101001\r\nContent-Disposition: form-data; name=\"file\"; filename=\"example-file\"\r\nContent-Type: application/octet-stream\r\n\r\n<string>\r\n-----011000010111000001101001--")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.maesn.dev/accounting/bookingProposals/{bookingProposalId}/attachments")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["X-API-KEY"] = '<x-api-key>'
request["X-ACCOUNT-KEY"] = '<x-account-key>'
request.body = "-----011000010111000001101001\r\nContent-Disposition: form-data; name=\"file\"; filename=\"example-file\"\r\nContent-Type: application/octet-stream\r\n\r\n<string>\r\n-----011000010111000001101001--"

response = http.request(request)
puts response.read_body

{
  "meta": {
    "warnings": [
      "Field not used by target system"
    ],
    "pagination": {
      "total": 125,
      "perPage": 50,
      "currentPage": 1,
      "totalPages": 3
    }
  },
  "data": [
    {
      "id": "document id",
      "base64Encoded": true,
      "content": "content",
      "contentType": "application/pdf",
      "fileName": "invoice.pdf"
    }
  ]
}