Having the attachments at the bottom is confusing. I would prefer the attachments to be at the top so that they're easier to see instead of hunting for them, particularly when you have a long e-mail thread...
Also, I noticed that attachments often don't open in the right program or the right program isn't available, such as in the case of WAV files where there is no option to play them from Thunderbird, but there is from other e-mail programs that I have and use.
When attachments are saved, I noticed that it sometimes adds irrelevant file extensions as well. For instance, when I saved a .wav file from my e-mail, Thunderbird saved it as "filename.wav.pdf" for some strange reason.
The biggest issue I see with Thunderbird currently is the handling of attachments, which doesn't seem to work very well.
I hope attachments can get some love in an upcoming release, because I can't really use Thunderbird as my e-mail client on mobile until I am able to work with attachments.
Thanks and I love Thunderbird otherwise.