For a picture I want to upload on last version of Discourse, max_image_size_kb and max_attachment_size_kb both at 4096, event if I set 20000 Kb, my picture (jpg) is 1.2Mo and I have this error:
Sorry, this file is too large (the maximum size allowed is 4096 KB).
Unless you can reproduce it here, this is a configuration problem with your instance. 4MB is the default upload limit in Discourse.
The most common cause of this issue is a reverse proxy that is not configured properly.
I had exactly the same problem, but setting
client_max_body_size in my reverse proxy fixed this.
But the error message displayed is misleading, because it tells the user the value set in
max_image_size_kb and not the setting that is causing the problem. Maybe this could be changed to a better wording.