It's a neat feature, and has saved me from forgetting to attach something many a time. When there isn't an attachment, one click tells it so.
It's sad that they've decided not to "maintain" it. There's nothing else comes close, in my view.
Unlike pesky Firefox: it updated this morning, and lost most of my settings. The tabs are now greyed out and at the top of the page, instead of in full contrast and immediately above the main box. Custom toolbar vanished, with all its icons and links. Forecastfox and the detailed weather stuff has gone. I think they want me to migrate to Chrome.