fix selectFile()

lib/utils/io.dart

@@ -160,11 +160,15 @@ class DirectoryPicker {
 Future<file_selector.XFile?> selectFile({required List<String> ext}) async {
   file_selector.XTypeGroup typeGroup = file_selector.XTypeGroup(
     label: 'files',
-    extensions: ext,
+    extensions: App.isMacOS || App.isIOS ? null : ext,
   );
   final file_selector.XFile? file = await file_selector.openFile(
     acceptedTypeGroups: <file_selector.XTypeGroup>[typeGroup],
   );
+  if (file == null) return null;
+  if (!ext.contains(file?.path.split(".").last)) {
+    return null;
+  }
   return file;
 }