When you right-click a file in Windows 11, the new context menu has a "Share" option that opens a panel with some apps you can send the file through, Thunderbird never appears there. TB should register itself as a Windows Share target so it shows up in that panel.
This is a relevant Microsoft article I think: https://learn.microsoft.com/en-us/windows/apps/develop/windows-integration/integrate-sharesheet-rece...
Thanks!