Managed file elements get wrapped + nested on each AJAX file upload

Problem/Motivation

When working with managed files in a regular form (opposed to using the field widget), the managed file elements gets wrapped on each file upload over and over again.

After uploading a file the AJAX return contains:

1. an optional error messages element (this is also the case for the field widget),
2. the AJAX wrapper and
3. an extra placeholder <span class="ajax-new-content"></span>.

AJAX returns that consist out of more than one element are being wrapped in a plain DIV by Drupal.ajax.prototype.commands.insert(). On the next file upload the AJAX wrapper is being replaced with another plain DIV element containing again an optional messages element, AJAX container and placeholder.

After a few uploads the results are multiple nested plain DIV's, each with an optional messages element and always with the placeholder. Though the nested plain DIV's and empty placeholders are probably invisible to the user (depending on the site's CSS), it does clutter up the DOM. The error messages that are never being removed is obviously more of a problem.

Proposed resolution

Alter file_ajax_upload() so elements are moved to inside the AJAX wrapper or stop inserting them to the DOM them at all.

The error messages are very much needed of course, the patch from #1792032-15: File validation error message not removed after subsequent upload of valid file moves them to inside the AJAX wrapper.

The placeholder could also be easily moved to the inside of the AJAX wrapper by not appending it to the #suffix property as in:

$form['#suffix'] .= '<span class="ajax-new-content"></span>';

but prepending it as in:

$form['#suffix'] = '<span class="ajax-new-content"></span>' . $form['#suffix'];

But... What is the use of the placeholder? .ajax-new-content elements are used by Drupal.ajax.prototype.commands.insert() to show / hide content that is being inserted by AJAX, but the placeholder is inserted empty and seems to never receive any content. Drupal.ajax.prototype.commands.insert() does not require such an element, so maybe we can remove this element from code completely?

Remaining tasks

Find out the use of the placeholder and remove it from code if there is none.

User interface changes

None.

API changes

None.