Elastic Beanstalk .ebextensions ignored in WAR

I am trying to change the client_max_body_size property of my Elastic Beanstalk NGINX reverse-proxy in order to allow uploads for larger JPEG files. Therefore, I added the folder ".ebextensions" to the root directory of my WAR file (the WAR file is also including a Spring Boot application) and added a file ".ebextensions/01_files.config" with the following content:

files:
  "/etc/nginx/conf.d/proxy.conf" :
    mode: "000755"
    owner: root
    group: root
    content: |
        client_max_body_size 20M;

I deploy the WAR file via Travis-CI to Elastic Beanstalk. However, it seems that the file is beeing ignored by Elastic Beanstalk since uploads with a filesize e.g. 2MB do not work and when connecting with SSH to the instance and looking for "/etc/nginx/conf.d/proxy.conf" the file does not exist.

I already successfully validated above content with an YAML validator. I know, there exists plenty of related questions but non of those seem to fix my problem. I also checked if ".ebextensions/01_files.config" is included in the WAR file in root directory. And when I check "/tmp/eb_extracted_jar", the file ".ebextensions/01_files.config" also exists with the correct content. I can't even find any errors in the "/var/log/cfn-init.log". I noticed that, just for some seconds, the file "proxy.conf" appeared in "/etc/nginx/conf.d/" during deployment but then it has been removed.

Can this problem occure because of the deployment to Elastic Beanstalk via Travis-CI? Or did I miss something else that is important?

EDIT: I just recognized that the "proxy.conf" file is created every time for a few seconds when the application is deployed but after a few seconds it disappears (checked with ls -lsa in "/etc/nginx/conf.d/", see the timestamps with 13:34 for "elasticbeanstalk" directory and "healthd_http.conf" and 13:43 for "proxy.conf")

4 drwxr-xr-x 3 root root 4096  6. Dec 13:43 .
4 drwxr-xr-x 4 root root 4096  6. Dec 13:34 ..
4 drwxr-xr-x 2 root root 4096  6. Dec 13:34 elasticbeanstalk
4 -rw-r--r-- 1 root root  148  6. Dec 13:34 healthd_http.conf
4 -rwxr-xr-x 1 root root   26  6. Dec 13:43 proxy.conf

And after a few seconds ls -lsa "/etc/nginx/conf.d/":

4 drwxr-xr-x 3 root root 4096  6. Dec 13:44 .
4 drwxr-xr-x 4 root root 4096  6. Dec 13:44 ..
4 drwxr-xr-x 2 root root 4096  6. Dec 13:44 elasticbeanstalk
4 -rw-r--r-- 1 root root  148  6. Dec 13:44 healthd_http.conf

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.