Horizon supports several different modes for uploading image files to Glance. The "legacy" mode that is used by default doesn't require any additional configuration, but has severe limitations and is not very efficient. On the other hand, the "direct" upload mode requires some additional configuration in Glance, and needs the Glance API endpoint to be exposed to the outside world.

Since we control both Horizon and Glance configurations, we can make sure that the "direct" upload mode is properly configured, and use that by default instead of the limited "legacy" mode. We need to cooperate with the Storage DFG to make sure it all works together.

Upstream documentation: https://docs.openstack.org/horizon/latest/configuration/settings.html#horizon-images-upload-mode