Issue with serving mbtiles

[UmeshLamture]

Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/gunicorn/workers/sync.py", line 135, in handle
self.handle_request(listener, req, client, addr)
File "/usr/local/lib/python2.7/dist-packages/gunicorn/workers/sync.py", line 176, in handle_request
respiter = self.wsgi(environ, resp.start_response)
File "/home/umesh/Downloads/TileStache-master/TileStache/init.py", line 413, in call
status_code, headers, content = requestHandler2(self.config, path_info, query_string, script_name)
File "/home/umesh/Downloads/TileStache-master/TileStache/init.py", line 286, in requestHandler2
status_code, headers, content = layer.getTileResponse(coord, extension)
File "/home/umesh/Downloads/TileStache-master/TileStache/Core.py", line 432, in getTileResponse
tile = self.render(coord, format)
File "/home/umesh/Downloads/TileStache-master/TileStache/Core.py", line 519, in render
tile = provider.renderTile(width, height, srs, coord)
File "/home/umesh/Downloads/TileStache-master/TileStache/MBTiles.py", line 234, in renderTile
return TileResponse(formats[mime_type], content)
KeyError: 'application/x-protobuf'

my config file is
{
"cache":
{
"name": "Test"
},
"layers":
{
"osm":
{
"provider":
{
"name": "mbtiles",
"tileset": "2017-07-03_france_monaco.mbtiles"
},
"preview":
{
"ext": "pbf"
}
}
}
}