How to create files route

import type { NextApiRequest, NextApiResponse } from "next";

const HOP_BY_HOP_HEADERS = new Set([
  "connection",
  "keep-alive",
  "proxy-authenticate",
  "proxy-authorization",
  "te",
  "trailer",
  "transfer-encoding",
  "upgrade",
]);

const CDN_BASE = "https://static-cdn.publive.online";
const ALLOWED_FILE_EXTENSIONS = new Set(["pdf", "doc", "docx", "xls", "xlsx", "csv", "txt", "zip"]);

export default async function handler(req: NextApiRequest, res: NextApiResponse) {
  const pathPart = req.query.path;
  const cdnPath = Array.isArray(pathPart) ? pathPart.join("/") : pathPart;

  if (!cdnPath) {
    res.status(404).send("Not Found");
    return;
  }

  const extension = cdnPath.split(".").pop()?.toLowerCase() || "";
  if (!ALLOWED_FILE_EXTENSIONS.has(extension)) {
    res.status(400).send("Only document/file routes are supported");
    return;
  }

  if (req.method !== "GET" && req.method !== "HEAD") {
    res.setHeader("Allow", "GET, HEAD");
    res.status(405).send("Method Not Allowed");
    return;
  }

  const upstreamUrl = new URL(`/${cdnPath}`, CDN_BASE);
  const queryString = req.url?.split("?")[1];
  if (queryString) upstreamUrl.search = queryString;

  const proxyHeaders = new Headers();

  for (const [key, rawValue] of Object.entries(req.headers)) {
    if (!rawValue) continue;

    const lowerKey = key.toLowerCase();
    if (HOP_BY_HOP_HEADERS.has(lowerKey)) continue;
    if (lowerKey === "host") continue;

    const value = Array.isArray(rawValue) ? rawValue.join(",") : rawValue;
    proxyHeaders.set(key, value);
  }

  proxyHeaders.set("host", upstreamUrl.host);

  const upstream = await fetch(upstreamUrl.toString(), {
    method: req.method,
    headers: proxyHeaders,
  });

  if (!upstream.ok && upstream.status >= 400) {
    res.status(upstream.status).send("Not Found");
    return;
  }

  upstream.headers.forEach((value, key) => {
    const lowerKey = key.toLowerCase();
    if (HOP_BY_HOP_HEADERS.has(lowerKey)) return;
    if (lowerKey === "content-length") return;
    res.setHeader(key, value);
  });

  res.setHeader("accept-ranges", "bytes");
  res.status(upstream.status);

  if (req.method === "HEAD") {
    res.end();
    return;
  }

  const body = await upstream.arrayBuffer();
  res.send(Buffer.from(body));
}

/** @type {import('next').NextConfig} */
const nextConfig = {
  async rewrites() {
    return [
      {
        source: "/files/:path*",
        destination: "/api/files/:path*",
      },
    ];
  },
};

module.exports = nextConfig;

export function toSameDomainFileUrl(url: string) {
  return url.replace("https://static-cdn.publive.online/", "/files/");
}