Now Client reads the first headers block, then LocalWebserver decides
which Handler to use. In case of "/upload", UploadFileHandler is used.
But now it only knows the "path" parameter. If that's valid, actual
UploadFileClientHandler is created, which reads the contents of the
request and, when finds there an "upload_file" field, starts saving it
in the directory specified by "path".
With that we don't need temp files approach from Reader class.
