How to attach file to email by copying files from HttpPostedFileBase to MemoryStream?

I'm using a custom SendEmailAfterSubmissionActor where i override the method Send. The reason if for attaching files to email when user is uploading files in my form.
CopyTo method in postFile.InputStream to MemoryStream i get this error.

Here is the full code for getting my files.

        private List<UploadedFile> GetUploadedFiles()
        {
            var uploadedFiles = new List<UploadedFile>();
            try
            {
                for (int i = 0; i < this.HttpRequestContext.Files.Count; i++)
                {
                    HttpPostedFileBase postedFile = this.HttpRequestContext.Files[i];
                    postedFile.InputStream.Position = 0;
                    postedFile.InputStream.Seek(0, SeekOrigin.Begin);
                    MemoryStream memoryStream = new MemoryStream();
                    postedFile.InputStream.CopyTo(memoryStream);
                    memoryStream.Position = 0;
                    memoryStream.Seek(0, SeekOrigin.Begin);
                    uploadedFiles.Add(
                        new UploadedFile()
                        {
                            Name = postedFile.FileName,
                            Type = MimeMapping.GetMimeMapping(postedFile.FileName),
                            InputStream = memoryStream
                        });
                }
            }
            catch (Exception ex)
            {
                PostSubmissionActorBase._logger.Error("Failed to get uploaded files: 
                     {0}", ex);
            }
            return uploadedFiles;
        }