fix uploading files

bin/server.dart

@@ -110,7 +110,7 @@ Future<void> _postOnMastodon(reqJson) async {
 
     for (final photoUrl in photoUrls) {
       final resp = await dio.post('$instanceUrl/api/v1/media',
-          data: dio_lib.FormData.fromMap({'file': File(photoUrl)}),
+          data: dio_lib.FormData.fromMap({'file': dio_lib.MultipartFile.fromFileSync(photoUrl)}),
           options:
               dio_lib.Options(headers: {'Authorization': 'Bearer $authToken'}));
       mediaIds.add(resp.data['id']);