How to strip off mail attachments in Thunderbird while keeping the mail

This is already implemented without addon:

just right-click on the attachment and choose "detach".

This will detach the attachment and keep the mail where it is.

There will be this message:

The following attachments have been successfully saved and will now be permanently deleted from this message: file.test

This action cannot be undone. Do you wish to continue?

Also, you can choose "delete" there.

If you want to extract attachments from lots of emails at once, you can use the add-on AttachmentExtractor (since Thunderbird 60 AttachmentExtractor Continued)

have a look at http://www.werockyourweb.com/thunderbird-mass-remove-attachments

there is a short introduction to the thunderbird-extension AttachmentExtractor. With that you can remove file attachements and choose what should happen to the remaining mail (archive, leave it or delete it...)