Alternatively, there is the built-in function vibe.http.fileserver.serveStaticFile(s). It's supposed to be used in conjunction with the router: 

auto router = new URLRouter;
router.get("/files/*", serveStaticFiles("./public/"));
router.get("/some/file.zip", serveStaticFile("./some/file.zip"));

It's also possible to use them from within a method handler, just with a syntax that is a little awkward: 

void downloadFile(HTTPServerRequest req, HTTPServerResponse res) {
    auto file = ...;
    serveStaticFile(file)(req, res);
}

The advantage of these functions is that they handle things like cache control (ETag/Last-Modified), HEAD requests, and directly stream the file instead of first reading it into RAM.

Hello Sir, the download now is much faster... one more thing how do I change the file name of the downloaded file? 

void downloadFile(HTTPServerRequest req, HTTPServerResponse res) {
 string title = db.select("title","books",req.params["book_id"]);
 string file = buildNormalizedPath("./data/inventory/"~req.params["book_id"]~".epub");
 scope(failure) res.writeBody("File Not Found");
 // check if there is a way to change the filename to title
 serveStaticFile(file)(req, res);
}