FIX Ensure DMSUploadField called via injection is returned correctly

Fixes an incompatibility with SelectUploadField in CWP

_config/config.yml

@@ -10,3 +10,7 @@ SiteTree:
 HtmlEditorField_Toolbar:
   extensions:
     - DocumentHtmlEditorFieldToolbar
+
+# Ensure that if the parent UploadField is overloaded with the injector, that the DMSUploadField is returned
+Injector:
+  DMSUploadField: DMSUploadField

tests/cms/DMSUploadFieldTest.php

@@ -14,6 +14,15 @@ class DMSUploadFieldTest extends SapphireTest
         $this->field = new DMSUploadField('StubUploadField');
     }
 
+    /**
+     * SS 3.x injector will return an overloaded parent of a child class if the child is not injected.
+     * This is a sanity check.
+     */
+    public function testDmsUploadFieldIsInjectable()
+    {
+        $this->assertInstanceOf('DMSUploadField', DMSUploadField::create('Stub'));
+    }
+
     /**
      * The validator is coded to always return true. Replace this test if this behaviour changes in future.
      */