add attachment icon to mail message in OS Monterey

It depends which view you're using. In Column Layout, right-click in the toolbar and select Attachments.

(You may have to drag the attachment column over from the far right of the window.)

In a non-column layout, the icons are there:

But they're not exactly easy to see - it's even worse in dark mode.

For me the paper clip still shows in the left pane where the emails are listed. It does not appear in the right pane with the opened mail message.

Control (right) - click on the header in a new message window and select Customize Toolbar. Then add the paperclip icon to the toolbar:

And again, in an open message window:

Not a triumph of user interface design...