From the main Administration Menu, click “Files”. Alternatively, you can select “Admin files” from the “Go to…” menu, located in the top right.
On the “Files” page, find the file you would like to categorize and click the “Sharing” button.
Set the file's category with the “File category” drop down.
Click “Save”.
The file will now be in the specified category on any page it should appear on.