Consuming an InfoPath document and a specified node path, decode all file attachments from the document and return them in a list of memory streams.

getInfoPathAttachments.cs

        public List<System.IO.MemoryStream> getInfoPathAttachments(System.Xml.XmlDocument InfoPathDocument, string nodePath)
        {
            /// <summary>
            /// Grab multiple attachments from an InfoPath form and
            /// return them in a list of memory streams.
            /// </summary>
            List<System.IO.MemoryStream> files = new List<MemoryStream>();
            System.Xml.XmlNamespaceManager nm = new System.Xml.XmlNamespaceManager(InfoPathDocument.NameTable);
            nm.AddNamespace("my", "http://schemas.microsoft.com/office/infopath/2003/myXSD/2014-07-12T15:29:09");

            //This needs to be the XML path to the node that contains the attachments.
            //In our case the attachments were litterally hanging off the base node.
            System.Xml.XmlNodeList xnl = InfoPathDocument.SelectNodes(nodePath, nm);
            //It is crucially important to note that we're using the SelectNodes() PLURAL method here!
            foreach (System.Xml.XmlNode nod in xnl)
            {
                string attachmentInBase64Text = nod.InnerText;
                byte[] attachmentByteArray = Convert.FromBase64String(attachmentInBase64Text);
                int fileNameBufferLength = attachmentByteArray[20] * 2;
                //Get the filename with extension.
                byte[] fileNameBuffer = new byte[fileNameBufferLength];
                for (int i = 0; i < fileNameBufferLength; i++)
                {
                    fileNameBuffer[i] = attachmentByteArray[24 + i];
                }
                string fileName = new string(UnicodeEncoding.Unicode.GetChars(fileNameBuffer));
                fileName = fileName.Substring(0, fileName.Length - 1);

                //A byte array for the file content.
                byte[] fileContent = new byte[attachmentByteArray.Length - (24 + fileNameBufferLength)];
                for (int i = 0; i < fileContent.Length; i++)
                {
                    fileContent[i] = attachmentByteArray[24 + fileNameBufferLength + i];
                }
                System.IO.MemoryStream ms = new MemoryStream(fileContent);
                files.Add(ms);
            }
            return files;
        }