One of my favorite features in Mailbird is attachment preview. It lets you take a look at an attached picture or PDF or .doc or whatever quickly, right inside Mailbird. You do not need to open word, paint, or any other apps that might be associated with the attachment, it's quite fast and convenient.
How to use it? Easy, you simply select any attachment that you want and press space bar on your keyboard. On the screenshot above I activate the quick preview from Mailbird attachment app. You can also do it from your email view (which you can see below) or anywhere else you can find an attachment. Please note that quick preview is only available for Mailbird Pro user.