There is no addon here PM Attachments.
My idea is this:
Addon PM Attachments will add more sociality to the forum.
Private message + image = this is interesting for users to communicate.
On SMF there is a mod PM Attachments https://custom.simplemachines.org/mods/index.php?mod=1974
Ask Dougie if he's ok with porting the code to Elk.
He uses a private license and I am not sure if he is willing to port that here. So far that I know, he had never been interested in ElkArte.
Sure, but he makes mods for free, so maybe he will if someone asks nicely. There's nothing to lose by asking, and given the code bases for SMF and Elk are so similar it's probably not hard to do.
If not, then by looking at how that mod was done it should be possible to code something that will do the same job. There will only be a couple of sane ways to do it.
Here I suggested the idea of an addon for Elk and showed an example of a working mod that I use on smf 2.0.
I do not know how all the mods from smf were ported to Elk. I have problems when I use Google translate from Russian to English and back. :)