There is an add-on that copies attachments from one message and attaches them to another, but you can also do it by dragging the attachment from the received message and dropping it in the area next to the addressing area in a new message.

@sfhowes Thank you for that, I downloaded and tried it, but when I copied a file to the clipboard and then tried to paste from clipboard, it said there was no file on the clipboard to attach. A quick perusal of the reviews for this extension show that it's not working for other users with v60 of TB.

I do know that I can drag and drop from one to another, but on a laptop with limited real estate, it's a tough sell. I'm trying to set this up for my aunt who was using Outlook previously and she enjoyed this right-click, copy and paste functionality from Outlook. I would love to find a way through TB as I think it's a superior product.

Here's how the add-on works: right-click an attachment, 'Copy in the clipboard for new message', then in a new message, click the Attach icon drop-down (or File/Attach), 'Attachments from other messages'. Selecting 'Files from the clipboard' works for a file copied from File Explorer (or other app). So, the language could be better, but it does work (once you know how).

There is an Attach from Clipboard add-on, but I don't see that it works with attachments to existing messages.