showOpenDialog with filter set doesn't show relevant dotfiles (microsoft#184592)

Fixes microsoft#182730

Author: alexr00

src/vs/workbench/services/dialogs/browser/simpleFileDialog.ts

@@ -994,9 +994,22 @@ export class SimpleFileDialog implements ISimpleFileDialog {
 		return sorted;
 	}
 
+	private extname(file: URI): string {
+		const ext = resources.extname(file);
+		if (ext.length === 0) {
+			const basename = resources.basename(file);
+			if (basename.startsWith('.')) {
+				return basename;
+			}
+		} else {
+			return ext;
+		}
+		return '';
+	}
+
 	private filterFile(file: URI): boolean {
 		if (this.options.filters) {
-			const ext = resources.extname(file);
+			const ext = this.extname(file);
 			for (let i = 0; i < this.options.filters.length; i++) {
 				for (let j = 0; j < this.options.filters[i].extensions.length; j++) {
 					const testExt = this.options.filters[i].extensions[j];