OK I finally got this to install! Major gripe - it does not resize attachments. Minor gripe - the settings would be better accessed from the TB menus rather than via Add-on Manager. After all the functionality being supplied is pretty mainstream usage.
This review is for a previous version of the add-on (126.96.36.199-signed).
I forgot that Thunderbird versions don't actually get released once they leave the beta channel, like they used to. I've now had a chance to check Shrunked against TB 17 and everything is fine, so the compatibility has been changed.