fix error

app/views/admin/announcements/_form_file.html.erb

@@ -52,7 +52,7 @@
         </li>
         <% end %>
       </ul>
-      <%= hidden_field_tag 'bulletin[bulletin_files_attributes][0][choose_lang][]', '' %>
+      <%= hidden_field_tag 'bulletin[bulletin_files_attributes][#{( form_file.new_record? ? 'new_bulletin_files' : "#{i}" )}][choose_lang][]', '' %>
     </span>
     <% if form_file.new_record? %>
       <span class="delete_file add-on btn" title="<%= t(:delete_) %>">
@@ -86,4 +86,4 @@
       <%= f.hidden_field :privacy_type, class: 'privacy_type' %>
     </span>
   </div>
-</div>
+</div>