ipn/localapi: serve files with application/octet-stream Content-Type

Updates tailscale/corp#7948

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I5f570c04974598c7abae4017e4a7a0f63492c87c
(cherry picked from commit 4caca8619e)

ipn/localapi/localapi.go

@@ -627,6 +627,7 @@ func (h *Handler) serveFiles(w http.ResponseWriter, r *http.Request) {
 	}
 	defer rc.Close()
 	w.Header().Set("Content-Length", fmt.Sprint(size))
+	w.Header().Set("Content-Type", "application/octet-stream")
 	io.Copy(w, rc)
 }