When using the in-app share button to share images, a file called "image_proxy" is shared, not the image itself. This started happening for me when I switched to lemmy.zip from lemm.ee - I just tested lemmy.zip in Thunder and it works as expected, so seems to be some combination of server+Boost causing the problem.
Jerboa has same problem. Seems to be crossposts causing issues but didn't dig into it.
Often the images themselves are called image proxy as well when I download it. This indicates the filename assigned to the downloaded image mirrors the url path instead of the filename it should be, which I am going to speculate is something like "[..]/image/image_proxy.php?filename=image_name"