File ID (fid) not migrated, differs from source

Hi,

Using this module to migrate Drupal 7 files into a Drupal 8 site, copying files from a remote HTTP source using the following piece of code:

source:
  plugin: d7_file
  scheme: public
  constants:
    source_base_path: sites/default/
    source_base_href: 'http://www.domain.org'
[...]
process:
  fid:
    -
      plugin: get
      source: fid
[...]
  source_remote_url:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/source_base_href
        - filepath
  file:
    -
      plugin: file_import
      source: '@source_remote_url'

This works quite well, except it does not take the old D7 fid into account, and generated a new fid based on the file_managed fid auto_increment. This will then result into other migrations not being able to lookup the file based on the old fid, resulting in missing files.

I will attach a patch that corrects this behaviour. This patch is WIP, since it's somewhat hard-coded, but at least illustrates the issue, and fixes it for now.