fs.go
func (fs *FS) NewRequestHandler() RequestHandler {
	fs.once.Do(fs.initRequestHandler)
	return fs.h
}
func (fs *FS) NewRequestHandler() RequestHandler {
	fs.once.Do(fs.initRequestHandler)
	return fs.h
}
// NewRequestHandler returns new request handler with the given FS settings.
//
// The returned handler caches requested file handles
// for FS.CacheDuration.
// Make sure your program has enough 'max open files' limit aka
// 'ulimit -n' if FS.Root folder contains many files.
//
// Do not create multiple request handlers from a single FS instance -
// just reuse a single request handler.