Since .NET Framework already have System.Net.Mime namespace and is able to parse Http headers, I hope there should be also "standard" or "hidden" method to extract mime body (base64 encoded) from message (passed as a string or stream).
Where?
P.S. now I use regexps but that is not so interesting end not so durable... P.P.S .Net Framework 3.5
I have not found any interesting inside .NET framework, after several hours of exploring with reflector.
2 I found that HTTP headers parser is also not very useful, have a look at
WebHeaderCollection's class,
internal DataParseStatus ParseHeaders(byte[] buffer, int size, ref int unparsed, ref int totalResponseHeadersLength, int maximumResponseHeadersLength, ref WebParseError parseError) method. It requires to know where to stop. That means preparsing.