As stated within our Modules to use with caution documentation, `dropzonejs` which is included with Acquia Cloud Lightning provides a feature known as `Bulk Upload` which allows for several files to be uploaded together. Some users will experience these uploads not completing correctly, with only a portion of the files uploaded showing up after completion.
in an environment which has multiple web servers which are being served by a single load balancer, the web server that processes the form submission may not be the web server that received the temporary file in the AJAX request, meaning that these files will not be stored correctly.
The simplest solution is to manually upload files through the
/media/add interface instead. Alternately, configure Drupal’s temporary files directory to use: