Set Email Attachment name in C#

I add an attachment like this:

System.Net.Mail.Attachment attachment = new System.Net.Mail.Attachment(AttachmentPath);   
msg.Attachments.Add(attachment);   

But I want to make it attach as a different name, the actual file name is very long and confusing I would like it to attach as "file.txt", is there an easy way to do this without having to make a copy of the file?

How about:

System.Net.Mail.Attachment attachment = new System.Net.Mail.Attachment(attachmentPath);
attachment.Name = "file.txt";  // set name here
msg.Attachments.Add(attachment);

You need to load the attachment from a stream and then you can give it a name and a media type.

var fs = new FileStream("attachmentPath", FileMode.Open);
var attachment = new System.Net.Mail.Attachment(fs, "MyAttachmentName.txt", "text/text");