Thanks to all.
So, I have Nginx in front of Discourse. It has configs:
/etc/nginx/nginx.conf (default)
/etc/nginx/conf.d/discourse.conf (custom)
Both configs have server{} sections. First I put client_max_body_size 100m; in custom config. Actually the setting did not work, but I did not check proxy logs.
When I moved client_max_body_size 100m; to default config (parent config), the problem resolved.
So, Changing Max Attachment Size article is actual. The root cause was in external proxy setting, outside of Discourse which is the best 
.