If you're getting this "Unknown property $ObjectType.File'?" message on the "Mass File Download" Tab then you have to check the Object list on the "About Mass File Download" tab.
Follow the Steps:
1. Go to the "About Mass File Download" tab
2. Scroll down and in the "Select List of Supported Objects" click on 'Refresh Objects' or Select the Object that you want to use for file/attachment download.
3. Now click on the "Save" button.
