Attributes key missing hash or pound sign in seven

Problem/Motivation

This error was reported by pookmish here: #3082690-127: Mark Media Library as a stable core module

Pookmish writes:

I believe this is the issue an error has come from:
User error: "0" is an invalid render array key in Drupal\Core\Render\Element::children()

I found that in the function seven_form_media_library_add_form_upload_alter() there is an incorrect attributes key.

$form['attributes']['class'][] = 'media-library-add-form--upload';

should be $form['#attributes']['class'][] = 'media-library-add-form--upload';

My database logs are full of this error now.

Proposed resolution

Add hash?

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet