How to upload string to ftp with flysystem and utf-8

Question

I have a task to upload a txt file to the FTP server using League\Flysystem with UTF-8

Code which is responsible for that is looking like this:

...
use League\Flysystem\FilesystemInterface;

final class UploadToFTPHandler implements CommandHandlerInterface
{
    private FilesystemInterface $filesystem;

    public function __construct(FilesystemInterface $ftpFilesystem)
    {
        $this->filesystem = $ftpFilesystem;
    }

    public function __invoke(UploadToFTP $command): void
    {
        $fileContents = 'którzy uczestniczyć w procesie o stary zamek, niegdyś własność';
        $fileContents = mb_convert_encoding($fileContents, 'UTF-8');

        $this->filesystem->write(
            sprintf('%s.txt', uniqid()),
            $fileContents,
            ['mimetype' => 'text/plain; charset=utf-8']
        );
    }
}

After uploading this file, when I open it on my server, it looks like this:

którzy uczestniczyć w procesie o stary zamek, niegdyĹ› wĹ‚asność

As you can see, there is no utf-8 encoding despite that I have set ['mimetype' => 'text/plain; charset=utf-8'] and mb_convert_encoding($fileContents, 'UTF-8')

What I am doing wrong to achieve correct UTF-8 encoding?

Implementation of this method $this->filesystem->write is here