BUG: Fix `MigrateFileTask` not migrating files for subsites

Fixes #352

_config/middleware.yml

@@ -8,3 +8,5 @@ SilverStripe\Core\Injector\Injector:
     properties:
       Middlewares:
         SubsitesStateMiddleware: %$SilverStripe\Subsites\Middleware\InitStateMiddleware
+  SilverStripe\Dev\Tasks\MigrateFileTask:
+    class: SilverStripe\Subsites\Tasks\SubsiteMigrateFileTask

src/Tasks/SubsiteMigrateFileTask.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace SilverStripe\Subsites\Tasks;
+
+
+use SilverStripe\Dev\Tasks\MigrateFileTask;
+use SilverStripe\Subsites\Model\Subsite;
+
+class SubsiteMigrateFileTask extends MigrateFileTask
+{
+    public function run($request)
+    {
+        $origDisableSubsiteFilter = Subsite::$disable_subsite_filter;
+        Subsite::disable_subsite_filter(true);
+
+        parent::run($request);
+
+        Subsite::disable_subsite_filter($origDisableSubsiteFilter);
+    }
+}