Thanks again for taking the time to test the add-on 
I think I added a reference to the Thunderbird documentation. The variables are all lower case there. But maybe this is too technical already. I will read through it again.
Suggestions to {{date}} and {{author}}: Both seem to be useful and not too difficult to implement. I think it would be good to group them together with the "Trim subject" setting. For now, the strings are directly taken from the Thunderbird API without any modification.
It's possible for sure, but I think there can be a few difficulties:
- There have to be two functions for parsing the image links: One for markdown mails and one for HTML mails.
- Error handling when there is no internet connection.
- Should the original inline image be replaced by the Joplin attachment? If yes, this would add some complexity, too.
This is an interesting topic, but will require some time. So I guess it won't happen soon.
I fear that the drag&drop behaves differently, depending on image format, target application and OS. It might be worth to search a bit through the forum or even open a new thread at the end. Maybe someone uses drag&drop regularly and can answer what works best and how it works.
As a side note: I think the recently discussed features/fixes won't be in the next release, because it needs to happen soon to stay compatible with Thunderbird 102. However, I'm happy to work on them and give a notice when there is something to show.