Blocking attachments by extension in Thunderbird

So after the third such incident at my work where a user had opened a .exe within a .zip, I decided to write an addon for Thunderbird that blocks attachments that are either executable or are zip files that contain an executable.

Our antivirus product (Kaspersky) has a feature to remove/rename .exe files within attachments, but unfortunately it can't remove/rename files within archives (I submitted an enhancement ticket for it).

Since we use shared hosting for our email we don't have control over the email server so I can't implement a filter directly on the email server which would have been the preferred solution.

So, my ultimate idea was a Thunderbird addon. Since this is my first such addon, and I had no idea how to write one, I forked another project and dropped my code into it as an addition.

Here is the source: https://github.com/glenritchie/SecondOpinion/tree/plusext

And pre-compiled(and signed) XPI files: https://github.com/glenritchie/SecondOpinion/tree/gh-pages/dist