Added extension to getBestPath API
Added extension to getBestPath API

Summary: Expand API by passing a default extension

Reviewed By: adityasharat

Differential Revision: D23078940

fbshipit-source-id: c9303add194e85deb174a8fc343e37b73f59d1c0

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper