curl --request POST \
  --url https://demo.komga.org/api/v1/books/{bookId}/thumbnails \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: multipart/form-data' \
  --form file=@example-file

import requests

url = "https://demo.komga.org/api/v1/books/{bookId}/thumbnails"

files = { "file": ("example-file", open("example-file", "rb")) }
headers = {"Authorization": "Basic <encoded-value>"}

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

print(response.text)

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

const options = {method: 'POST', headers: {Authorization: 'Basic <encoded-value>'}};

options.body = form;

fetch('https://demo.komga.org/api/v1/books/{bookId}/thumbnails', 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://demo.komga.org/api/v1/books/{bookId}/thumbnails",
  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 => [
    "Authorization: Basic <encoded-value>",
    "Content-Type: multipart/form-data"
  ],
]);

$response = curl_exec($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://demo.komga.org/api/v1/books/{bookId}/thumbnails"

	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("Authorization", "Basic <encoded-value>")

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

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

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://demo.komga.org/api/v1/books/{bookId}/thumbnails")
  .header("Authorization", "Basic <encoded-value>")
  .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://demo.komga.org/api/v1/books/{bookId}/thumbnails")

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

request = Net::HTTP::Post.new(url)
request["Authorization"] = 'Basic <encoded-value>'
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

{
  "bookId": "<string>",
  "fileSize": 123,
  "height": 123,
  "id": "<string>",
  "mediaType": "<string>",
  "selected": true,
  "type": "<string>",
  "width": 123
}

{
  "violations": [
    {
      "fieldName": "<string>",
      "message": "<string>"
    }
  ]
}