Set Content-Disposition with encoded filename

ContentDisposition.cs

public static void SetContentDispositionInline(this HttpResponse response, string fileName, string userAgent)
{
    SetContentDisposition(response, "inline", fileName, userAgent);
}

public static void SetContentDispositionAttachment(this HttpResponse response, string fileName, string userAgent)
{
    SetContentDisposition(response, "attachment", fileName, userAgent);
}

private static void SetContentDisposition(this HttpResponse response, string dispositionType, string fileName, string userAgent)
{
    response.AddHeader("Content-Disposition", $"{dispositionType}; filename=\"{EncodeContentDespositionFileName(fileName, userAgent)}\"");
}

private static string EncodeContentDespositionFileName(string fileName, string userAgent)
{
    if (!string.IsNullOrEmpty(fileName))
    {
        var fileNameEcoding = Encoding.UTF8;

        // Microsoft Internet Explorer or Edge
        if (!string.IsNullOrEmpty(userAgent)
            && ((userAgent.IndexOf("Edge", StringComparison.OrdinalIgnoreCase) >= 0)
                || (userAgent.IndexOf("MSIE", StringComparison.OrdinalIgnoreCase) >= 0)
                || (userAgent.IndexOf("Trident", StringComparison.OrdinalIgnoreCase) >= 0)
                || (userAgent.IndexOf("rv:11.0", StringComparison.OrdinalIgnoreCase) >= 0)))
        {
            fileNameEcoding = Encoding.GetEncoding("windows-1251");
        }

        var fileNameBytes = fileNameEcoding.GetBytes(fileName);

        var dispositionFileName = "";

        foreach (var b in fileNameBytes)
        {
            dispositionFileName += (char)(b & 0xff);
        }

        return dispositionFileName;
    }

    return fileName;
}

Example:

response.SetContentDispositionAttachment("Файл.pdf", requestUserAgent);

where requestUserAgent you can take from the request header.