Customised: Migrate File dataobjects from 3.x and successive iterations in 4.x
Imports all files referenced by File dataobjects into the new Asset Persistence Layer introduced in 4.0.
Moves existing thumbnails, and generates new thumbnail sizes for the CMS UI.
Fixes file permissions.
If the task fails or times out, run it again and it will start where it left off.
You need to flush your cache after running this task via CLI.
See https://docs.silverstripe.org/en/4/developer_guides/files/file_migration/.