There are two ways to add attachments to a message in Airmail, either by “Drag & Drop” (the easiest method) or by selecting an attachment on your computer using Finder and adding it to the message.
Drag & Drop
To use this method, open a new message window (or reply to an existing message) and drag and drop any attachment and it will show in the bottom section of the composer. If you drag and drop images and want them as attachments rather than inline images, drag them and release below the horizontal indicator line in the bottom section of the composer, as shown in the picture below.
Please also see how to attach 'Inline Images'?
Attaching via Finder
Alternatively, you can use Finder to locate a file or image on your Mac and add it to the Composer window. To do this, click on the 'paper clip icon' at the top of the Composer window. This will open a Finder window so you can select your file and it will be added to your message.
The attachments will show in the bottom section and the message size will be shown in the title bar on the top.