Facebook group problem: how to post files

Hi, please can someone help. Some Facebook groups I belong to have a ‘More’ option just to the right of ‘live video’ option above the text area for posting, with ‘add file’ in the drop down. This allows members to post a document file, such as Doc, RTF or PDF to the group. The group I administer does not. How do I change settings to make it happen? I have hunted everywhere, without success. Thanks.