diff --git a/docs/en/04_Changelogs/4.0.0.md b/docs/en/04_Changelogs/4.0.0.md
index a8a342958..d37364e52 100644
--- a/docs/en/04_Changelogs/4.0.0.md
+++ b/docs/en/04_Changelogs/4.0.0.md
@@ -817,7 +817,7 @@ large amounts of memory and run for an extended time.
 
 
 ```yml
-File:
+SilverStripe\Assets\File:
   migrate_legacy_file: true
 ```
 
@@ -828,11 +828,21 @@ 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.
 
+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.
 
-
 #### Upgrade code which acts on `Image`
 
 As all image-specific manipulations has been refactored from `Image` into an `ImageManipulations` trait, which