无法将fileupload.postedfile.inputstream作为附件发送-错误:无法将system.io.stream转换为字符串

using System;
using System.IO;
using System.Net.Mail;

// 读取文件流并发送附件
public void SendAttachment(Stream fileStream)
{
    // 将文件流转换为字节数组
    byte[] fileBytes;
    using (var memoryStream = new MemoryStream())
    {
        fileStream.CopyTo(memoryStream);
        fileBytes = memoryStream.ToArray();
    }

    // 将字节数组转换为Base64编码的字符串
    string base64String = Convert.ToBase64String(fileBytes);

    // 创建邮件对象
    var mail = new MailMessage();
    mail.From = new MailAddress("sender@example.com");
    mail.To.Add(new MailAddress("recipient@example.com"));
    mail.Subject = "附件邮件";
    mail.Body = "这是一封带有附件的邮件";

    // 创建附件对象
    var attachment = new Attachment(new MemoryStream(fileBytes), "attachment.txt");
    attachment.ContentDisposition.Inline = false;
    attachment.ContentDisposition.DispositionType = DispositionTypeNames.Attachment;

    // 将Base64编码的字符串作为附件内容
    attachment.ContentStream = new MemoryStream(Convert.FromBase64String(base64String));

    // 添加附件到邮件对象
    mail.Attachments.Add(attachment);

    // 发送邮件
    var smtpClient = new SmtpClient("smtp.example.com");
    smtpClient.Send(mail);
}