Broken email attachments (files, images) with webform

Issue

Email attachments uploaded via webform are broken, including images or files, with the following error message:

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)

Resolution

Try downgrading the Memcache module to version 8.x-2.1

Cause 

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 and you can track the issue or comment on the public thread here: https://www.drupal.org/project/memcache/issues/2996615