File attachments cannot be removed via quickedit

File attachments cannot be removed via quick edit. Tested on a fresh 8.3.7 install.

Steps to reproduce:

1. Create a new “File” field on a content-type with default settings.
2. Create a new node for this content-type, successfully upload a .txt file.
3. Save the node.
4. On the node full view use quickedit and try to remove the file.

Results:

- The file is not removed
- The console shows the following errors:
Failed to load resource: the server responded with a status of 500 (Internal Server Error) http://drupal-8.app/quickedit/form/node/2/field_another_file/en/full?element_parents=field_another_file/widget/0&_wrapper_format=drupal_ajax&ajax_form=1&_wrapper_format=drupal_ajaxFailed to load resource: the server responded with a status of 500 (Internal Server Error)

AjaxError: 
An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /quickedit/form/node/2/field_another_file/en/full?element_parents=field_another_file/widget/0&_wrapper_format=drupal_ajax&ajax_form=1
StatusText: Internal Server Error
ResponseText: {}

- The Drupal PHP log shows the following errors:
LogicException: The database connection is not serializable. This probably means you are serializing an object that has an indirect reference to the database connection. Adjust your code so that is not necessary. Alternatively, look at DependencySerializationTrait as a temporary solution. in Drupal\Core\Database\Connection->__sleep() (line 1471 of /var/www/drupal-8/web/core/lib/Drupal/Core/Database/Connection.php).