For now to attach files, the workaround is to read the input stream directly from this.HttpRequestContext.Files and attach the stream in the mail message.

Here is an idea how I did it:

        public override object Run(object input)
        {
            /* The file uploading might take a while so we have to wait till Epi Forms successfully uploads the files. */
            Task task = Task.Factory.StartNew(() =>
            {
                GetUploadedFiles();
            });
            task.Wait();

            // This is inherited code of base class
            IEnumerable<EmailTemplateActorModel> emailModel = this.Model as IEnumerable<EmailTemplateActorModel>;
            if (emailModel == null || emailModel.Count<EmailTemplateActorModel>() < 1)
                return (object)null;

            this.SendMessage(emailModel);

            return (object)null;
        }

        private IList<UploadedFile> GetUploadedFiles()
        {
            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 = System.Web.MimeMapping.GetMimeMapping(postedFile.FileName),
                            InputStream = memoryStream
                        }
                    );
                }
            }
            catch (Exception ex)
            {
                PostSubmissionActorBase._logger.Error("Failed to get uploaded files: {0}", ex);
            }
            return uploadedFiles;
        }

    public class UploadedFile
    {
        public string Name { get; set; }
        public string Type { get; set; }
        public MemoryStream InputStream { get; set; }
    }

+1 -- helps with upholding GDPR