Does anyone know about this？
The Android app in chat can only view thumbnails and not the original images, but they are normal in the web、iOS and desktop apps. I have turned off the “Enable image thumbnails to save bandwidth” and the auto-rotation of images.
Server version.：4.8.1
app version：4.29.0.34199
Hi! Welcome to our community 
Does it happen only at Android? While using browser, iOS or desktop app does it work properly?
Also, how is your reverse proxy configured?
Yes， only android, I’m using the reverse proxy that comes with Synology NAS, I’m not a professional developer, so I’m not sure if I’ve configured it correctly.
Also, I can’t view pictures properly when I send them on my phone, but they work fine in system albums and other apps.
Also，this is a super long picture，size：1080*23712
but does it happen with small pictures too?
This is most likely something with the reverse proxy.
I have seen some weird issues, maybe like this one, when the firewall/reverse proxy enforces TLS 1.3
This is our doc for reverse proxy. You will need to try replicating those configurations, for example, for nginx, so we can maybe isolate the problem.
Also, if you could, try using nginx instead synology NAS, even if as a test, so we can check if the issue is indeed at reverse proxy level.
Thanks, I will try learning to use nginx at the weekend, Small images will be displayed correctly， but maybe your image magnification is set too low, what is your image magnification limit? 
Maybe you can try this image you can view on a mobile device?
Because I use the ios device is iPad and not iPhone, I think this is related to the screen size.
wow!
that’s a “huge” image and certainly a very edge case scenario.
Sorry, this has nothing to do with nginx. I was not considering the dimensions you said. 
This is most certainly a bug.
I will create an Issue here after poking our devs