Addon doesn't remove attacment from the message

This addon still works in Thunderbird 52.9, but it doesn't remove attachments from the message.
Previously reported on addons since Feb. 2016
https://addons.thunderbird.net/en-GB/thunderbird/addon/webdav-for-filelink/reviews/

Start by reproducing the attachment-removal failure in the WebDAV Filelink addon on Thunderbird 52.9, then trace the addon’s message and attachment handling entry point. Done means removing an attachment through the addon also removes it from the message, with the behavior verified in Thunderbird.

Written by the indexing model from the issue text.