You can only 'file' a document once all the required fields are completed. Therefore, the File button will only appear in the tool bar in your In Tray if you have filled out the filing details in the FYI drawer for all the selected document(s).
Refer to Filing Multiple Emails/Documents in Managing your In Tray.
Required fields are determined by any settings for the selected Cabinet settings which are not "Optional". Refer to Managing Cabinets.