Inline images lost in sent email

I have upgraded to RT 5.0. We have long had problems with inline images in RT 4.x, but I’m revisiting this in case there’s a solution I’m missing.

If I send an email from Thunderbird to RT (via rt-mailgate) with inline images, it displays fine in the RT web interface. However, the email sent to AdminCcs etc has the structure corrupted and the image is attached but not inlined.

If I reply with an inline image, the same happens.

Is there a solution to this? There have been patches posted here before but I never got this to work.