File import not working

Thanks for the feedback. These are the steps i took.

I went to this link https://www.drupal.org/docs/8/upgrade/upgrade-using-drush and followed the instruction.

First of all i run the code below

drush migrate-upgrade --legacy-db-url=mysql://user:password@server/db --legacy-root=http://example.com --configure-only

Which generated the all the migration scripts and this is the output of the migrate_plus.migration.upgrade_d7_file.yml

langcode: en
status: true
dependencies: {  }
id: upgrade_d7_file
migration_tags:
  - 'Drupal 7'
migration_group: migrate_drupal_7
label: Files
source:
  plugin: d7_file
  scheme: public
  constants:
    source_base_path: ''
process:
  fid: fid
  filename: filename
  source_full_path:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/source_base_path
        - filepath
    -
      plugin: urlencode
  uri:
    plugin: file_copy
    source:
      - '@source_full_path'
      - uri
  filemime: filemime
  status: status
  created: timestamp
  changed: timestamp
  uid: uid
destination:
  plugin: 'entity:file'
  source_base_path: var/www/www.example.com/
migration_dependencies:
  required: {  }
  optional: {  }

from the upgrade_d7_file it doesn't make sense to me why the "source_base_path: var/www/www.example.com/" should be under destination instead of the source.

When i run the command
drush migrate-import --limit=10 upgrade_d7_file
I got this feedback
Processed 10 items (0 created, 0 updated, 10 failed, 0 ignored) - done with 'upgrade_d7_file'
So to debug i run following command to show me what happened
drush mmsg upgrade_d7_file
and this was the feedback but all the files listed below are available on my drupal 7 site

 source_ids_hash          level                   message
 61498ee526a129         1            File '/sites/default/files/book.png' does not exist
 90304713aa4be5         1            File '/sites/default/files/meet.gif' does not exist
 eacaf701d2256ad         1            File '/sites/default/files/time_0.png' does not exist
 6716890b1f27075        1            File '/sites/default/files/tJan2_0.gif' does not exist
 ef65baa6be4aae6         1            File '/sites/default/files/over.png' does not exist
 032cf7e48cf066ca        1            File '/sites/default/files/meetJohn.gif' does not exist
 a2b77d2477e3249       1            File '/sites/default/files/Family.gif' does not exist
 68a955a65238116       1            File '/sites/default/files/Now.gif' does not exist
 247b62dc96c9ffd2        1           File '/sites/default/files/Problem.gif' does not exist
 f2acf3ffb908e372e        1           File '/sites/default/files/Solution.gif' does not exist

Is there something more i need to add to my script or what am i missing in order to be able to successfully migrate files

I finally got it to work, i had to update my drupal core to 8.4.2 and also update the migrate_plus, migrate_tools, migrate_upgrade modules to make this work

Secondly

1. if you are migrating files from a d7 live website to a d8 local/live site use --legacy-root=http://example.com
2. if you are migrating files from a d7 local website to a d8 localsite use --legacy-root=/var/www/drupal/

I hope this helps someone

Awesome Solution