Migrating files and images
This page currently describes migration of File entities. Drupal 8 core also has Media entities which require specific migrations
The file migration can be done in two separate migrations as follows:
First migrate the files using entity:file destination plugin.
Use migration_lookup process plugin in the node migration to associate the previously migrated files to the nodes.
An alternative and much simpler approach is to use the contributed Migrate Files (extended) module which provides process plugins for migrating images and files from an internal or external source and a third process plugin for migrating remote files so that they are used from the remote location.