附件类中system.net.mime.mediatypenames的意义是什么?

MailMessage msg = new MailMessage();
string fileName = "C:\Test\Sample.pdf";
Attachment attach = new Attachment(fileName, System.Net.Mime.MediaTypeNames.Application.pdf);
msg.Attachments.Add(attach);

MailMessage msg = new MailMessage();
string fileName = "C:\Test\Sample.pdf";
Attachment attach = new Attachment(fileName);
msg.Attachments.Add(attach);

添加mime类型的意义是什么?如果我不指定一个陷阱/缺点,有没有什么陷阱/缺点?