constructor(filePath) {
        this.filePath = filePath;
        this.headers = new Headers();

        this.exists = fs.existsSync(filePath);
        if (this.exists) {
            this.status = 200;
            this.statusText = 'OK';

            let stats = fs.statSync(filePath);
            this.headers.set('content-length', stats.size.toString());

            this.updateContentType();

            const stream = fs.createReadStream(filePath);
            this.body = new ReadableStream({
                start(controller) {
                    stream.on('data', (chunk) => controller.enqueue(chunk));
                    stream.on('end', () => controller.close());
                    stream.on('error', (err) => controller.error(err));
                },
                cancel() {
                    stream.destroy();
                }
            });
        } else {
            this.status = 404;
            this.statusText = 'Not Found';
            this.body = null;
        }
    }
Creates a new `FileResponse` object.
@param {string} filePath