drupal/core/modules/file/migrations/d6_file.yml

# Every migration that references a file by Drupal 6 fid should specify this
# migration as an optional dependency.
id: d6_file
label: Public files
audit: true
migration_tags:
  - Drupal 6
  - Content
source:
  plugin: d6_file
  constants:
    # The tool configuring this migration must set source_base_path. It
    # represents the fully qualified path relative to which URIs in the files
    # table are specified, and must end with a /. See source_full_path
    # configuration in this migration's process pipeline as an example.
    source_base_path: ''
process:
    # If you are using both this migration and d6_user_picture_file in a custom
    # migration and executing migrations incrementally, it is strongly
    # recommended that you remove the fid mapping to avoid potential ID
    # conflicts. For that reason, this mapping is commented out by default.
    # fid: fid
  filename: filename
  source_full_path:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/source_base_path
        - filepath
    -
      plugin: urlencode
  destination_full_path:
    plugin: file_uri
    source:
      - filepath
      - file_directory_path
      - temp_directory_path
      - is_public
  uri:
    plugin: file_copy
    source:
      - '@source_full_path'
      - '@destination_full_path'
  filemime: filemime
  # No need to migrate filesize, it is computed when file entities are saved.
  # filesize: filesize
  status: status
  changed: timestamp
  uid: uid
destination:
  plugin: entity:file