While reviewing the changes, I came to the conclusion that this needs a deeper API change to work efficiently with the non-class streams of vibe-core. What I would propose is to drop the MultiPart(BodyPart)' class(es) completely and change the signature of writeParttovoid writePart(InputStream)(InputStream content, in ref InetHeaderMap headers = emptyHeaderMap) if (isInputStream!InputStream)`. That would avoid the need for a proxy that does dynamic dispatch and destroys all inlining possibilities.

Of course there can still be the convenience overloads that take other kinds of types for content.