Applescript to import mail with attachments into devonthink that would expand archived files
I can see two options:

First is to save the file and simply use the open command like is it shown at the link below. I tried it and it works well enough.

https://stackoverflow.com/questions/13651533/create-an-applescript-or-shell-script-to-unzip-and-rename-files

Alternatively, you can use a do shell script command to use the unzip command line tool to do the same thing like shown here,

https://discussions.apple.com/message/9062795#9062795

But extracting the archive contents is the easy part. Does the archive have a structure? Will it always be the same? Here is where you may run into trouble.

Hope this helps.