2. Upload an image in the wysiwyg editor
3. Console error: Uncaught TypeError: Cannot read property 'length' of undefined in Drupal.file.Drupal.file.validateExtension
Because that template, or more specifically template_preprocess_file_managed_file(), adds an id based on #element['id'], which is also added to the actual input element.
Add a suffix to the id? Just remove it (what does it even do?)
Speaking of that, what's the point of this template? And why does it exist in an identical version in classy and file?