SendGrid附件错误:无法从“系统”转换。伊奥。MemoryStream'到'string'

我在azure存储中有PDF格式的文件。现在我想把它作为附件附在邮件中。对于邮件发送,我使用SendGrid 9.8.0.0版。但它给出了类似“无法从”系统转换的错误。伊奥。MemoryStream'到字符串“”。

代码如下:

CloudBlockBlob blob = container.GetBlockBlobReference(fileName);

            var stream = new MemoryStream();
            blob.DownloadToStream(stream);
            stream.Seek(0, SeekOrigin.Begin);
            ContentType content = new ContentType(MediaTypeNames.Text.Plain);
            stream.Position = 0;

            #endregion

            var msg = new SendGridMessage()
            {
                From = new EmailAddress(email.FromMail),
                Subject = email.Subject,
                HtmlContent = email.MailBody,

            };

            msg.AddAttachment(stream, "originalfilename.png"); <-- here giving error
            msg.AddTo(new EmailAddress(email.Recipient));

这是怎么回事??