diff --git a/docs/en/02_Developer_Guides/14_Files/04_File_Migration.md b/docs/en/02_Developer_Guides/14_Files/04_File_Migration.md
new file mode 100644
index 000000000..e0cf8cd9a
--- /dev/null
+++ b/docs/en/02_Developer_Guides/14_Files/04_File_Migration.md
@@ -0,0 +1,57 @@
+summary: Manage migration of legacy files to the new database structure
+
+# File migration
+
+Since the structure of `File` dataobjects has changed between 3.0 and 4.0, a new task `MigrateFileTask`
+has been added to assist in migration of legacy files.
+
+You can run this task on the command line:
+
+```
+$ ./framework/sake dev/tasks/MigrateFileTask
+```
+
+This task will also support migration of existing File DataObjects to file versioning. Any
+pre-existing File DataObjects will be automatically published to the live stage, to ensure
+that previously visible assets remain visible to the public site.
+
+If additional security or visibility rules should be applied to File dataobjects, then
+make sure to correctly extend `canView` via extensions.
+
+## Automatic migration
+
+Migration can be invoked by either this task, or can be configured to automatically run during dev build
+by setting the `File.migrate_legacy_file` config to true. However, it's recommended that this task is
+run manually during an explicit migration process, as this process could potentially consume large
+amounts of memory and run for an extended time.
+
+```yml
+SilverStripe\Assets\File:
+  migrate_legacy_file: true
+```
+
+## Migration of thumbnails
+
+If you have the [asset admin](https://github.com/silverstripe/silverstripe-asset-admin) module installed
+this will also ensure that thumbnails for these images are generated when running 'MigrateFileTask'.
+Existing thumbnails will not be migrated however, and must be re-generated for use in the CMS.
+
+Note: Thumbnails can be regenerated on a one-by-one basis within the CMS by re-saving it
+within the file edit details form.
+
+## Discarded files during migration
+
+Note that any File dataobject which is not in the `File.allowed_extensions` config will be deleted
+from the database during migration. Any invalid file on the filesystem will not be deleted,
+but will no longer be attached to a dataobject anymore, and should be cleaned up manually.
+
+To disable this, set the following config:
+
+```yaml
+SilverStripe\Assets\FileMigrationHelper:
+  delete_invalid_files: false
+```
+
+Note that pre-existing security solutions for 3.x (such as
+[secure assets module](https://github.com/silverstripe/silverstripe-secureassets))
+are incompatible with core file security.
diff --git a/docs/en/04_Changelogs/4.0.0.md b/docs/en/04_Changelogs/4.0.0.md
index 487621855..0c5859b90 100644
--- a/docs/en/04_Changelogs/4.0.0.md
+++ b/docs/en/04_Changelogs/4.0.0.md
@@ -829,24 +829,16 @@ guide for configuration instruction.
 
 #### Migrating File DataObject from 3.x to 4.0
 
-Since the structure of `File` dataobjects has changed, a new task `MigrateFileTask` has been added to assist
-in migration of legacy files. Migration can be invoked by either this task, or can be configured to automatically
-run during dev build by setting the `File.migrate_legacy_file` config to true. However, it's recommended that
-this task is run manually during an explicit migration process, as this process could potentially consume
-large amounts of memory and run for an extended time.
+Since the structure of `File` dataobjects has changed between 3.0 and 4.0, a new task `MigrateFileTask`
+has been added to assist in migration of legacy files.
 
+You can run this task on the command line:
 
-```yml
-SilverStripe\Assets\File:
-  migrate_legacy_file: true
+```
+$ ./framework/sake dev/tasks/MigrateFileTask
 ```
 
-This task will also support migration of existing File DataObjects to file versioning. Any
-pre-existing File DataObjects will be automatically published to the live stage, to ensure
-that previously visible assets remain visible to the public site.
-
-If additional security or visibility rules should be applied to File dataobjects, then
-make sure to correctly extend `canView` via extensions.
+For more information on this task please consult the [file migration documentation](/developer_guides/files/file_migration).
 
 Note that any File dataobject which is not in the `File.allowed_extensions` config will be deleted
 from the database during migration. Any invalid file on the filesystem will not be deleted,
@@ -859,10 +851,6 @@ SilverStripe\Assets\FileMigrationHelper:
   delete_invalid_files: false
 ```
 
-Note that pre-existing security solutions for 3.x (such as
-[secure assets module](https://github.com/silverstripe/silverstripe-secureassets))
-are incompatible with core file security.
-
 #### Upgrade code which acts on `Image`
 
 As all image-specific manipulations has been refactored from `Image` into an `ImageManipulations` trait, which
diff --git a/src/Dev/Tasks/MigrateFileTask.php b/src/Dev/Tasks/MigrateFileTask.php
index 7b19c5b82..4b026b876 100644
--- a/src/Dev/Tasks/MigrateFileTask.php
+++ b/src/Dev/Tasks/MigrateFileTask.php
@@ -2,6 +2,7 @@
 
 namespace SilverStripe\Dev\Tasks;
 
+use SilverStripe\AssetAdmin\Helper\ImageThumbnailHelper;
 use SilverStripe\ORM\DB;
 use SilverStripe\Assets\FileMigrationHelper;
 use SilverStripe\Dev\BuildTask;
@@ -21,11 +22,23 @@ class MigrateFileTask extends BuildTask
 
     public function run($request)
     {
+        if (!class_exists(FileMigrationHelper::class)) {
+            DB::alteration_message("No file migration helper detected", "notice");
+            return;
+        }
+
         $migrated = FileMigrationHelper::singleton()->run();
         if ($migrated) {
             DB::alteration_message("{$migrated} File DataObjects upgraded", "changed");
         } else {
             DB::alteration_message("No File DataObjects need upgrading", "notice");
         }
+
+        if (!class_exists(ImageThumbnailHelper::class)) {
+            DB::alteration_message("No image thumbnail helper detected", "notice");
+            return;
+        }
+
+        ImageThumbnailHelper::singleton()->run();
     }
 }