Did you remove the old version before installing the fixed version? What is your version of TB?

I have TB 68.10.0 (64bit). I'm not aware of having an old version installed. I didn't know about LookOut before yesterday.

For TB 68, install v. 2.04:

https://addons.thunderbird.net/en-US/thunderbird/addon/lookout-fix-version/versions/2.0.4

It could not be installed because it can't be verified.

edit: it has not been verified

It looks like you're clicking on the Download button and trying to install in Firefox. Drag the green Download Now button and drop it onto Tools/Add-ons in TB.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

I was unable to complete drag and drop between the two windows. I took the other option given in the link. I saved the Download link to my computer, then installed it in Thunderbird. It seems to be installed. Do I need to do anything else to have it running? Can I expect future attachments that would otherwise have said "winmail.dat files can't be opened" to now open? Thank you for your help.

As I recall from using it once to decode a winmail.dat attachment, it makes such attachments appear as regular files with the underlying extension.