Derive datasource type from 'External' if possible.

lib/carto/renderer.js

@@ -98,6 +98,9 @@ carto.Renderer = function Renderer(env) {
                                 next(new Error('No .shp file found in zipfile'));
                             } else {
                                 l.Datasource.file = file;
+                                l.Datasource.type = l.Datasource.type ||
+                                    (external.type.ds_options &&
+                                    external.type.ds_options.type);
                                 next();
                             }
                         });