这个问题通常是由于在构建邮件时未正确处理邮件附件所导致的。以下是一个示例代码，展示了如何在Asp.net core中发送包含附件的电子邮件，并正确处理附件：

public async Task SendEmailAsync(string recipientEmail,
                                        string subject,
                                        string message,
                                        List attachments = null)
{
    try
    {
        var email = new MimeMessage();
        email.From.Add(new MailboxAddress(_emailConfig.FromName, _emailConfig.FromAddress));
        email.To.Add(new MailboxAddress(recipientEmail));
        email.Subject = subject;

        var builder = new BodyBuilder();
        builder.HtmlBody = message;
        
        // Attachments
        if (attachments != null && attachments.Count > 0)
        {
            foreach (var attachmentPath in attachments)
            {
                if (!File.Exists(attachmentPath))
                    continue;
                
                builder.Attachments.Add(attachmentPath);
            }
        }
        
        email.Body = builder.ToMessageBody();

        using (var client = new SmtpClient())
        {
            await client.ConnectAsync(_emailConfig.SmtpServer, _emailConfig.Port, SecureSocketOptions.StartTls);
            await client.AuthenticateAsync(_emailConfig.UserName, _emailConfig.Password);
            await client.SendAsync(email);
            await client.DisconnectAsync(true);
        }
        
        return true;
    }
    catch (Exception ex)
    {
        // Handle exception here
        return false;
    }
}

在上述代码中，我们正确地处理了邮件的附件，可以使用以下示例方法调用：

await SendEmailAsync("recipient@example.com", "Test Email", "Test Email", new List { "test.txt", "image.jpg" });

在这个示例中，我们将包含在List参数中的文件路径添加到邮件主体中。请注意，如果filepath不存在，则会跳过相应的附件。