File server: respond to HEAD adequately

[vemv]

It's usual to curl -I (HEAD request) to check whether an endpoint is alive. FW's file server returns a spurious 404.

I don't terribly need this but it would be a nice addition to prevent surprises!

Cheers - Victor

[bhauman]

Maybe this should be pushed over to the compojure project bc that's the code serving these files.

Need to investigate more.