MIME is a protocol for sending data, graphics, etc via email.
Any email client/server should support it in order to transport mail.
Decoding is done by the mail client, like MS Outlook. You have to automate Outlook to get it to open encoded files.

If you want to write C code to read mime:

There are RFC's about the standardization of the protocol.

http://www.faqs.org/rfcs/rfc2045.html

http://www.faqs.org/rfcs/rfc1522.html

http://www.faqs.org/rfcs/rfc1521.html