Email attachments uploaded via webform are broken: images, files, ...
Warning: file_get_contents(private://webform/test/_sid_/1.jpg): failed to open stream: "Drupal\Core\StreamWrapper\PrivateStream::stream_open" call failed in Drupal\webform\Plugin\WebformElement\WebformManagedFileBase->getAttachments() (line 1483 of /mnt/www/html/<subscription>/docroot/modules/contrib/webform/src/Plugin/WebformElement/WebformManagedFileBase.php)
You need to downgrade Memcache module to version 8.x-2.1
Cause (this is optional)
As you can see in the error message, the `_sid_` variable is not replaced by its value, therefore the path is invalid.
It's due to this issue with Cache Tags invalidation : https://www.drupal.org/project/memcache/issues/2996615