Access denied errors by server itself on uploaded files?
I have discovered that the delete-this files are temporary files created by CiviCRM for the sole purpose of generating this error! This error is good, it means that these directories are not browseable, and they shouldn't be browseable.

CiviCRM's CRM_Utils_Check_Security class does these checks in order to report what would-be security errors.

It creates a file, then tries to access that file via http. This should fail.