In Thunderbird, the icons for attachments in messages are pretty huge. If the big size offends your design sensibilities, you can configure Thunderbird to use smaller icons.

To do this, you need to get to your preferences editing panel. (On the Mac, you do this by going to Thunderbird > Preferences... in the menu bar. Not sure about other operating systems). Once you get the preferences panel, select the Advanced pane, and then the General tab. Near the bottom right will be a button called Config Editor.... Click it. This will open up about:config editor for Thunderbird.

Once you are there, scroll down to the preference called mailnews.attachments.display.largeView and double click on it. This will change the value from true to false and then Thunderbird will used smaller sized icons.