For security reasons Opigno LMS using a private file system to check if a user has related permission to download a particular file. For example, answers for "Opigno file upload" activity type should be accessible only for users who create these answers and users with global role "Content manager" etc.
The private files system is not configured out of the box so need to be done these steps to configure it:
To manage the allowed extensions for the "File upload" activity, you will have to edit these 2 config files:
and then re-import them (at /admin/config/development/configuration/single/import)