Rated 5 out of 5 stars

Works a treat!
Despite some other reviews, this worked perfectly for me (I'm using version 91.7.0)
There is some room for improvement (e.g. if the list of attachments is too long, the 'OK' or 'Cancel' buttons fall off the end of the screen and you can't access them. This is easily remedied by selecting no more than about 30 mails at once. The add-on gives you the option of deleting the attachment from the mail or not, so there's no fear of losing data if you want the attachment retained in your mails.

This review is for a previous version of the add-on (1.4).