Rendering mbtiles on OSM

Question

I am new to maps. I want to render mbtiles created using TileMill on open street map using Leaflet. My question is 1)How to render it on maps? As of now I am extracting the images using this command mb-util Fair3.mbtiles ~/Documents/fair3 and then rendering on map using leaflet with following code.
L.tileLayer('static/fair3/{z}/{x}/{y}.png', { attribution: '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors' }).addTo(map); The problem here is after certain zoom level the tiles are not showing up and there is background(bounding box) which i want it to be transparent.

2)If I upload fair3.mbtiles to Mapbox, how to render tiles from my Mapbox account?