Warning: Undefined array key "extension" in Drupal\stage_file_proxy\DownloadManager->writeFile() (line 202 of modules/contrib/stage_file_proxy/src/DownloadManager.php).

Think we need more info than this. Need steps to reproduce the problem.

Probably need a different fix to handle missing files then just an isset. This may get the warning to go away but probably masking a larger bug