As the owner or moderator of a group, you can prevent users of the group from including HTML or attachments in messages they send to the group. To do so:
1. From any page within the group, click the "Management" link at the bottom of the left hand column navigation.
2. Then click "Messages" in the "Groups Setting" section.
3. Then click the "edit" link to the "Postings & Archives" section.
4. Then choose the "Remove" button in the "Attachments" column within "Posting Options".
5. Then "Save Changes".
However, choosing this option does not mean that users of the group cannot receive the new email format. Whether or not to receive this format is an individual decision that each user makes themselves, and cannot be made for the group by the group owner.