fix attachment size

lib/src/file_attachment.dart

@@ -4,10 +4,12 @@ import 'package:stream_chat_flutter/src/utils.dart';
 
 class FileAttachment extends StatelessWidget {
   final Attachment attachment;
+  final Size size;
 
   const FileAttachment({
     Key key,
     @required this.attachment,
+    this.size,
   }) : super(key: key);
 
   @override
@@ -18,8 +20,8 @@ class FileAttachment extends StatelessWidget {
           launchURL(context, attachment.assetUrl);
         },
         child: Container(
-          width: 100,
-          height: 100,
+          width: size?.width ?? 100,
+          height: size?.height ?? 100,
           child: Center(
             child: Icon(Icons.attach_file),
           ),