Suspicious behaviour of Spring Web Application - rest

Question

I was auditing my spring web app's security and found a strange thing. Whenever I try to hit the address https://xxxxxxxxx.xxx/app then browser captures a document for downloading. But there is no API named "/app" is listed on my REST controller. Moreover, the document is blank.
Here is my request info:
GET /app HTTP/1.1
Host: xxxxxxxx.xxx
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-GB,en;q=0.5
Accept-Encoding: gzip, deflate
Cookie: NG_TRANSLATE_LANG_KEY=%22en%22; count=0
Connection: close
Upgrade-Insecure-Requests: 1
Here is response sent from the server:
HTTP/1.1 200 OK
Server: nginx/1.13.12
Date: Tue, 05 Jun 2018 11:19:01 GMT
Content-Type: application/octet-stream
Content-Length: 0
Connection: close
Expires: Sun, 05 Jun 2022 11:19:01 GMT
Cache-Control: max-age=126230400000, public
X-XSS-Protection: 1; mode=block
Pragma: cache
Accept-Ranges: bytes
Last-Modified: Fri, 01 Jun 2018 08:50:14 GMT
X-Content-Type-Options: nosniff
X-Application-Context: some-app
Whenever I try it from my local system then there is no issue like this. I've already disabled the directory listing for my application but the problem is still there. Please let me know if any other information is required.
My NGINX conf is as follows:
server {
listen 443 ssl http2;
server_name xxxxxxxx.xxx;
# Configure SSL
ssl_certificate /etc/ssl/certs/nginx/xxxxxx.xxx.chained.crt;
ssl_certificate_key /etc/ssl/certs/nginx/xxxxxxx.key;
include /etc/nginx/includes/ssl.conf;
location / {
include /etc/nginx/includes/proxy.conf;
proxy_pass http://10.210.xx.xx:8080;
}
access_log off;
error_log /var/log/nginx/error.log error;
}