var binaryData = File.ReadAllText(@"E:teststream.txt");
byte[] byteArray = Convert.FromBase64String(binaryData);
File.WriteAllBytes(@"E:testfile.pdf", byteArray);

byte[] data;

data = File.ReadAllBytes(@"E:teststream.txt");
string base64 = System.Text.Encoding.ASCII.GetString(data);
data = Convert.FromBase64String(base64);
File.WriteAllBytes(@"E:testfile.pdf", data);

var attachments = (from a in cc.CRMContext.ActivityMimeAttachments where a.ActivityId == email.ActivityId select a).ToList();
                foreach (var attachment in attachments)
                {
                    byte[] byteArray = Convert.FromBase64String(attachment.Body);
                    System.IO.File.WriteAllBytes(@"E:testtemp" + attachment.FileName, byteArray);
                }

var binaryData = File.ReadAllText(@"E:teststream.txt");
byte[] byteArray = Convert.FromBase64String(binaryData.Replace(' ', '+'));
File.WriteAllBytes(@"E:testfile.pdf", byteArray);