fix serving static webmail files in development mode

due to a missing return, the content was served again.
this path doesn't happen on release binaries, only during local development,
where there is a local file that can be served.

webmail/webmail.go

@@ -311,6 +311,7 @@ func serveContentFallback(log *mlog.Log, w http.ResponseWriter, r *http.Request,
 		st, err := f.Stat()
 		if err == nil {
 			http.ServeContent(w, r, "", st.ModTime(), f)
+			return
 		}
 	}
 	http.ServeContent(w, r, "", fallbackMtime(log), bytes.NewReader(fallback))