Prevent inline attachment also showing up as external attachment

This happens in some email clients (gmail below).

We want to fix the image from showing up as a separate email attachment, which is in fact the behavior in some email clients already.

<img width="948" height="238" alt="Image" src="https://github.com/user-attachments/assets/f904c502-3690-45ea-a628-e60f02f267ba" />