https://help.yahoo.com/kb/SLN2543.html
If you're a group owner or moderator with privileges to change group settings, there are multiple settings available for privacy and membership approval in your group. Just click Management | Control Panel to adjust any of these options:
Group Settings—Change privacy settings, joining and viewing settings, photomatic settings, email preferences, and how attachments are distributed:
Plain Text
HTML - No Attachments
HTML - Include Attachments as Links
HTML - Include Attachments in Email