C# - cannot attach file to an email

I have a service which creates Excel file and sends by email.

So I have a strange trouble. This service works correctly on a test server but on a production server I have a bug - there is strange file in attachments without extension with name like this "=utf-8B0J3QtdC0YDQvtGB0YLQsNCy0LvQtdC9" and the same content inside insted of a excel file.

There is my sending method. I suppose that there are trubles not in my code. I would be very grateful if somebody can help me.

public static void SendEmail(string subject, string body, string recipients, Attachment attachment = null)
    {
        string mailServer = Settings.Default.MailServer;

        MailMessage message = new MailMessage(Settings.Default.MailSender, recipients);
        foreach (string address in Settings.Default.MailCopyTo.Split(',').Select(x => x.Trim()))
        {
            if (!string.IsNullOrEmpty(address))
                message.CC.Add(new MailAddress(address));
        }
        message.IsBodyHtml = true;
        message.Body = body;
        message.Subject = subject;

        if(attachment != null)
            message.Attachments.Add(attachment);


        SmtpClient client = new SmtpClient(mailServer);
        var AuthenticationDetails = new NetworkCredential(Settings.Default.MailLogin, Settings.Default.MailPassword);
        client.Credentials = AuthenticationDetails;
        client.Send(message);
    }