Add new features and update configurations for improved functionality

lib/ui/models/attachment.dart

@@ -0,0 +1,22 @@
+import 'dart:typed_data';
+
+class Attachment {
+  final String name;
+  final String mimeType;
+  final Uint8List data;
+  final DateTime createdAt;
+
+  Attachment({
+    required this.name,
+    required this.mimeType,
+    required this.data,
+    DateTime? createdAt,
+  }) : createdAt = createdAt ?? DateTime.now();
+
+  bool get isImage => mimeType.startsWith('image/');
+  bool get isPdf => mimeType == 'application/pdf';
+  bool get isText => mimeType.startsWith('text/');
+
+  int get sizeInKB => (data.length / 1024).ceil();
+  String get displayName => name;
+}