add await for file download

bin/server.dart

@@ -79,14 +79,14 @@ Future<void> _postOnMastodon(reqJson) async {
   final photos =
       attachments.where((attachment) => attachment['type'] == 'photo');
 
-  final photoUrls = photos.map((photo) {
+  final photoUrls = photos.map((photo) async {
     final photoUrl = photo['photo']['sizes'].last['url'];
     // Generate a random filename.
     // Add the extension '.jpg'.
     final photoFilename = '${DateTime.now().millisecondsSinceEpoch}-${photoUrl.split('/').last.split('?').first.substring(0, 10)}.jpg';
     final photoPath =
         '${Directory.systemTemp.path}/$photoFilename';
-    dio.download(photoUrl, photoPath);
+    await dio.download(photoUrl, photoPath);
     return photoPath;
   });