Thank you, got it.
Working as expected.
Every single one of those images is marked with:
Content-Disposition: inline
And the message's top-level structure is:
Content-Type: multipart/related
This -- clearly and unambiguously -- means that the images are not attachments, but are embedded somewhere in message text (HTML).
They're actually not referenced anywhere in message text, but they are marked as such.
The developer of this Firefox add-on should spend some time reading the relevant documentation (RFCs), easy to find on the Internet. Or he could just send a message with a quality mail app or web mail (like Thunderbird or Gmail), and examine the message's source.
Or you could just forward him this:
For a message with attachments, the top-level structure should be:
Content-Type: multipart/mixed
And each attachment should have:
Content-Disposition: attachment
PS - thought I'd add something about "but it works with other apps".
There is a reason why there are technical standards for all kinds of things, including email messages.
It lets apps work with each other, without having to add workarounds in every app for every other app it might interact with.