File To Bas64String Convert

https://msdn.microsoft.com/ru-ru/library/dhx0d524(v=vs.110).aspx

https://stackoverflow.com/questions/25919387/c-sharp-converting-file-into-base64string-and-back-again

https://stackoverflow.com/questions/25919387/c-sharp-converting-file-into-base64string-and-back-again/25919641

https://social.msdn.microsoft.com/Forums/en-US/e5965e02-0bbb-4f8d-9e5f-8611aa902457/how-to-convert-pdf-file-to-base64binary?forum=csharpgeneral

https://developer.salesforce.com/forums/?id=906F00000009As4IAE