private async Task<string?> GetPreSignedUrlAsync(DocumentKey key, bool inline, CancellationToken cancellationToken)
{
    if (!await ObjectExistsAsync(key, cancellationToken))
        return null;

    var request = new GetPreSignedUrlRequest
    {
        BucketName = _settings.S3.Bucket,
        Key = key.ToString(),
        Verb = HttpVerb.GET,
        Expires = DateTime.UtcNow.Add(DownloadUrlLifetime),
        ResponseHeaderOverrides =
        {
            ContentDisposition = inline
                ? InlineContentDisposition(key.Name.ToString())
                : AttachmentContentDisposition(key.Name.ToString()),
        },
    };

    return await s3.GetPreSignedURLAsync(request);
}

private static string AttachmentContentDisposition(string fileName)
{
    var header = new ContentDispositionHeaderValue("attachment");
    header.SetHttpFileName(fileName);
    return header.ToString();
}

/// <summary>
/// Builds a <c>Content-Disposition</c> header that lets the browser render the document inline
/// while preserving the file name, using RFC 5987 encoding so non ASCII file names survive.
/// </summary>
private static string InlineContentDisposition(string fileName)
{
    var asciiFallback = new string(fileName.Select(c => c is >= ' ' and < (char)127 and not '"' ? c : '_').ToArray());
    var encoded = Uri.EscapeDataString(fileName);
    return $"inline; filename=\"{asciiFallback}\"; filename*=UTF-8''{encoded}";
}