Elastic Beanstalk: .ebextensions not copied over

Currently I am trying to extend nginx configuration for my EB instance. I read through this support document but the config file in .ebextensions is not executed and my additional nginx configuration is not created in the EB instance.

.ebextensions/proxy.config

files:
  /etc/nginx/conf.d/proxy.conf:
    mode: "000644"
    owner: root
    group: root
    content: |
      http {
        client_max_body_size 100M;
      }

(also tried with 000755)

Dockerrun.aws.json

{
  "AWSEBDockerrunVersion": "1",
  "Image": {
    "Name": "***.dkr.ecr.eu-central-1.amazonaws.com/***:***",
    "Update": "true"
  },
  "Ports": [
    {
      "ContainerPort": "3000"
    }
  ]
}

.elasticbeanstalk/config.yml

branch-defaults:
  develop:
    environment: ***
    group_suffix: null
  ...
deploy:
  artifact: artifact.zip
global:
  application_name: ***
  branch: null
  default_ec2_keyname: ***
  default_platform: Docker running on 64bit Amazon Linux 2
  default_region: eu-central-1
  include_git_submodules: true
  instance_profile: null
  platform_name: null
  platform_version: null
  profile: null
  repository: null
  sc: git
  workspace_type: Application

I deploy and check as follows:

zip -r artifact.zip .ebextensions Dockerrun.aws.json
eb deploy
ls /etc/nginx/conf.d/

But I cannot see proxy.conf there, so it's not copied during the deployment.

Folder structure in the zip file is as follows:

/Dockerrun.aws.json
/.ebextensions/proxy.config