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:
upload-7d396c29-2da4-472a-b51c-f17bb5301066
Image used to create this error:

Discover the magic of the internet at Imgur, a community powered entertainment destination. Lift your spirits with funny jokes, trending memes, entertaining gifs, inspiring stories, viral videos, and so much more from users.
To resolve:
In 'nginx's server' context of enabled site config
client_max_body_size 20M;
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:
https://community.nodebb.org/topic/5404/image-upload-on-android
Thanks,
Drew