#151 use path.extname for data file ext. extraction

src/data.preview.ts

@@ -149,7 +149,7 @@ export class DataPreview {
     this._dataViews = (views !== undefined) ? views: {};
     this._viewConfig = viewConfig;
     this._fileName = path.basename(uri.fsPath);
-    this._fileExtension = this._fileName.substr(this._fileName.lastIndexOf('.'));
+    this._fileExtension = path.extname(this._fileName); // file extension
     this._previewUri = this._uri.with({scheme: 'data'});
 
     // parse view config