Yes, I am sure, or I wouldn't have mentioned it. Not everyone uses Thunderbird. (I use Mail.app for OS X and roundcube webmail. Both display them as attachments.)

tbh, I don't pay attention to priority ever, since the only people who use it when they email me are spammers.

So the moral is: not everyone uses the same client as you, and not everyone uses email the same way as you. To piss off the least number of people, go for a lowest common denominator thing. This is also why I always use plain text email.