KML is a file format that is like GeoJSON, but used more commonly in Google products. Like GeoJSON, it can store points, lines, polygons, and other vector data. Unlike GeoJSON, it's based on XML, rather than JSON.
When uploading KML, please note that Mapbox does not support any KML extensions. For more information on uploading shapefiles, check out the Mapbox Studio Manual uploads page.Download a sample KML file