Steps to reproduce:
- Open moodle mobile app and input the site "https://sandbox.moodledemo.net/"
- Login by teacher account
- Go to "My first course"
- Go to Forum “News forum” > Add a new discussion topic
- Click “Advanced” > Click "Add file" > Choose attachment exceeds maxsize
- Observe the display of screen
The issue is that it should show a warning that you can't attach the file because it is too big, but actually, it does show the file as if it will be included (it won't be really).