Initial things together; however, going to rework some more.

modules/dgi_migrate_foxml_standard_mods/migrations/dgis_tn_file.yml

@@ -66,11 +66,11 @@ process:
     - plugin: dgi_migrate.method
       source: '@_latest'
       method: getUri
-  _path:
-    - plugin: format_date
-      source: '@created'
-      from_format: U
-      to_format: 'Y-m'
+#  _path:
+#    - plugin: format_date
+#      source: '@created'
+#      from_format: U
+#      to_format: 'Y-m'
   filemime:
     - plugin: dgi_migrate.subproperty
       property: MIMETYPE
@@ -89,19 +89,22 @@ process:
         - '@_safe_pid'
         - '@_ext'
       delimiter: '.'
-  _dest_uri:
-    - plugin: concat
-      source:
-        - constants/file_dest
-        - '@_path'
-        - '@filename'
-      delimiter: '/'
+#  _dest_uri:
+#    - plugin: concat
+#      source:
+#        - constants/file_dest
+#        - '@_path'
+#        - '@filename'
+#      delimiter: '/'
+#  uri:
+#    - plugin: dgi_migrate.naive_file_copy
+#      file_exists: rename
+#      source:
+#        - '@_source_uri'
+#        - '@_dest_uri'
   uri:
-    - plugin: dgi_migrate.naive_file_copy
-      file_exists: rename
-      source:
-        - '@_source_uri'
-        - '@_dest_uri'
+    - plugin: dgi_migrate.foxml_file
+      pid:
   filesize:
     - plugin: callback
       source: '@uri'

src/Plugin/migrate/process/FoxmlFile.php

@@ -0,0 +1,108 @@
+<?php
+
+namespace Drupal\dgi_migrate\Plugin\migrate\process;
+
+use Drupal\Component\Transliteration\TransliterationInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Plugin\MigrateProcessInterface;
+use Drupal\migrate\Plugin\MigrationInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * General FOXML file handling plugin.
+ *
+ * Accepts:
+ * - method: One of "copy" (to copy the file) or "direct" (to directly use the
+ *   file). Defaults to "copy". Can be set with the DGI_MIGRATE_FILE_METHOD
+ *   environment variable.
+ * - pid: Property in the row containing the PID, to build out a destination if
+ *   copying.
+ * - destination: Property in the row containing the destination, to build out
+ *   a destination path if copying.
+ * - date: Property in the row containing a date with which to build a path in
+ *   the destination, when copying.
+ * - mimetype; Property in the row containing the MIME type of the file, to
+ *   determine an extension.
+ *
+ * @MigrateProcessPlugin(
+ *   id = "dgi_migrate.foxml_file"
+ * )
+ */
+class FoxmlFile extends ProcessPluginBase {
+
+  /**
+   * Constructor.
+   */
+  public function __construct(
+    array $configuration,
+    $plugin_id,
+    $plugin_definition,
+    protected MigrationInterface $migration,
+    protected TransliterationInterface $transliteration,
+    protected MigrateProcessInterface $naiveCopyPlugin,
+    protected MigrateProcessInterface $extensionPlugin,
+  ) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
+    $configuration['method'] ??= getenv('DGI_MIGRATE_FILE_METHOD') ?: 'copy';
+    assert(in_array($configuration['method'], ['copy', 'direct']));
+    /** @var \Drupal\migrate\Plugin\MigratePluginManagerInterface $process_plugin_manager */
+    $process_plugin_manager = $container->get('plugin.manager.migrate.process');
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $migration,
+      $container->get('transliteration'),
+      $process_plugin_manager->createInstance('dgi_migrate.naive_file_copy'),
+      $process_plugin_manager->createInstance('dgi_migrate.process.extension_from_mimetype'),
+    );
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    return match ($this->configuration['method']) {
+      'copy' => $this->naiveCopyPlugin->transform([
+        $value,
+        $this->getDestinationPath($migrate_executable, $row, $destination_property),
+      ], $migrate_executable, $row, $destination_property),
+      'direct' => $value,
+    };
+  }
+
+  /**
+   * Build out path.
+   */
+  protected function getDestinationPath(MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) : string {
+    $pid = $row->get($this->configuration['pid']);
+    // Adapted from https://api.drupal.org/api/drupal/core%21modules%21migrate%21src%21Plugin%21migrate%21process%21MachineName.php/class/MachineName/10
+    $safe_pid = preg_replace(
+      '/_+/',
+      '_',
+      preg_replace(
+        '/[^a-z0-9_]+/',
+        '_',
+        strtolower(
+          $this->transliteration->transliterate($pid, LanguageInterface::LANGCODE_DEFAULT, '_'),
+        ),
+      )
+    );
+
+    $dest_dir = $row->get($this->configuration['destination']);
+    $path = date('Y-m', $row->get($this->configuration['date']));
+    $ext = $this->extensionPlugin->transform($row->get($this->configuration['mimetype']), $migrate_executable, $row, $destination_property);
+
+    return "{$dest_dir}/{$path}/{$safe_pid}.{$ext}";
+  }
+
+}