If you are using a reverse proxy with nginx and attempt to upload an image over certain images the nginx server refuses the file with its own message rather than the expected nodebb response.
Screenshot from this forum:
Image used to create this error:
View post on imgur.com
In 'nginx's server' context of enabled site config
Hope that helps this forum and others using reverse proxy
If you're in the mood for helping me, then I've raised this bug report for a separate issue: