implemented file thumbnails and enhanced file type icons, added reusable FileLeading widget, and updated search to support previews

lib/view/pages/files/widgets/file_element.dart

@@ -16,6 +16,7 @@ import '../../../../widget/details_bottom_sheet.dart';
 import '../../../../widget/info_dialog.dart';
 import '../../talk/widgets/highlighted_linkify.dart';
 import 'file_details_sheet.dart';
+import 'file_leading.dart';
 
 class FileElement extends StatefulWidget {
   final CacheableFile file;
@@ -372,9 +373,7 @@ class _FileElementState extends State<FileElement> {
 
   @override
   Widget build(BuildContext context) => ListTile(
-    leading: CenteredLeading(
-      Icon(widget.file.isDirectory ? Icons.folder : Icons.description_outlined),
-    ),
+    leading: CenteredLeading(FileLeading(file: widget.file)),
     title: _title(context),
     subtitle: _subtitle(),
     trailing: Icon(widget.file.isDirectory ? Icons.arrow_right : null),