var mimePart = (attachment as MimePart);
// Decode to file
 using (var fileStream = new FileStream("c:Temp", FileMode.Create, FileAccess.Write))
            {
                mimePart?.ContentObject.DecodeTo(fileStream);
            }

// Upload to S3
using (FileStream fileToUpload =
                new FileStream("C:TempTheFile", FileMode.Open, FileAccess.Read))
            {
                fileTransferUtility.Upload(fileToUpload,
                                           "bucket-name", "key");
            }

using (FileStream fileToUpload = (FileStream)mimePart.ContentObject.Stream)
            {
                fileTransferUtility.Upload(fileToUpload,
                                           "bucket-name", "key");
            }