Problems with file migration

I’m in the process of migrating a fairly large old site that has been migrated from Drupal 6 to Drupal 7, and now to Drupal 8.

There are a few issues about getting this done, but I have a grip on most of it. (I’m tossing a lot of functionality; in some cases the migrations are failing, but that’s mostly okay.)
The BIG problem I have is that I cannot for the life of me get the file migrations to work. The Drupal 7 site has both public and private files. I shall leave the private files for the time being. I cannot even get the public files to move over. My first stab was at moving them through http:// so I have set the source to https://wis.community, but the Drupal 8 site never seems to touch the Drupal 7 website, and just fails on every file with an error message similar to the following one.

‘/sites/default/files/images/151659850_3902971153115469_2314013813948011350_o.png’ does not exist

The other website never gets hit. I’ve also tried changing the configuration by editing the d7_file.yml file and re-importing it, but that constantly fails the same way. It appears that, no matter HOW I try to set up the file migration, it doesn’t work.

I’m obviously missing something basic here but I don’t get it.
The migrate_plus.migration.upgrade_d7_file.yml config file looks like this.

uuid: da623100-323b-473f-9a67-5d6776cf586b
langcode: en
status: true
dependencies: {  }
id: upgrade_d7_file
class: DrupalmigratePluginMigration
field_plugin_method: null
cck_plugin_method: null
migration_tags:
  - 'Drupal 7'
  - Content
migration_group: migrate_drupal_7
label: 'Public files'
source:
  plugin: d7_file
  scheme: public
  constants:
    source_base_path: 'https://wis.community/'
process:
  fid:
    -
      plugin: get
      source: fid
  filename:
    -
      plugin: get
      source: 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:
    -
      plugin: get
      source: filemime
  status:
    -
      plugin: get
      source: status
  created:
    -
      plugin: get
      source: timestamp
  changed:
    -
      plugin: get
      source: timestamp
  uid:
    -
      plugin: get
      source: uid
  field_metatag:
    -
      plugin: d7_metatag_entities
      source: pseudo_metatag_entities
destination:
  plugin: 'entity:file'
migration_dependencies:
  required: {  }
  optional: {  }

How can I resolve this issue?