You can modify a file type group by clicking the name of that group on the File Type Groups Setup page. This brings up the File Type Group:name page. You can click Add to add new entries. When you finish adding entries, the information is added as new table rows.
You can click any link in the MIME Type column to bring up the Edit File Type Group Entry window and to modify the information in that row. You can delete a row by selecting the row and clicking Delete above the table. When you finish, click Save to save your changes.