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).