Elastic Beanstalk .ebextensions ignored in WAR
After hours of reading docs, I found out that I missed some important parts of the official AWS docs for the Elastic Beanstalk Java SE Platform (see http://docs.aws.amazon.com/elasticbeanstalk/latest/dg/java-se-platform.html).
Definitely my mistake: I used the wrong file extension, the file extension in the folder ".ebextensions" has to be ".conf" but not ".config".
At least for the Java SE Platform: one can directly add NGINX config files within the ".ebextensions" directory without using the "files: ..." syntax to generate a file with a specific content, i.e. to create the proxy file in "/etc/nginx/conf.d/proxy.conf" just add ".ebextension/nginx/conf.d/proxy.conf" with the content
client_max_body_size 20M;directly. Subsequently, "proxy.conf" will then be deployed to "/etc/nginx/conf.d/proxy.conf" and automatically included by the default NGINX config.
Hope this answer saves someone else the time it took me to figure that out.