There is no need to "convert it into original base64" — so called "attachment" is an e-mail part usually encoded in base64 already (or it could be just plain text, for text parts), in a multi-part messages. Open any mail block with a plain text editor and see by yourself.
Please see these CodeProject articles: http://www.codeproject.com/search.aspx?doctypeid=1&q=pop3+mail+"C%23"
Even if you don't use POP3, you will find the rest of the solution, the one you are missing: how to parse the multipart mail message and extract parts. This way, you can extract "attachments".
And your "does not work" is not informative — if you still cannot resolve it, show your code using "Improve question" above.