[Bug] Download path generation invalid in subdir behind nginx

Description:

A valid upload file cannot be downloaded, because the download link provided differs from the actual upload link when Rocket.Chat runs in a subdir. Seems like RC is adding the already proxied subdir a second time for downloads.

Worked in pre 3.12 versions - started occuring with v3.12.0.

May be related to #20640, but can't figure out if the issue is the same or if there is just an invalid apache/nginx configuration in #20640.

Steps to reproduce:

1. Install RC as docker with a subdir e.g. /chat
2. Run RC behind a native nginx (config below)
3. Upload a file
4. Try to download the file

Expected behavior:

The file is downloaded successfully

Actual behavior:

A dummy file is created with a few kB instead.

Server Setup Information:

• Version of Rocket.Chat Server: 3.12.0
• Operating System: Ubuntu Server
• Deployment Method: docker
• Number of Running Instances: 1
• DB Replicaset Oplog: wiredTiger
• NodeJS Version: 12.18.4 - x64
• MongoDB Version: 4.0.18

Client Setup Information

• Desktop App or Browser Version: applies to both
• Operating System: Windows/macOS/Linux

Additional context

Comparing upload and download links reveals the issue. A manual change of the download link (removing the invalid second /chat/ fixed it.

Upload Link:
https://<ROOT-URL>/chat/file-upload/8KAZmCZcswoEWQcZt/<FILENAME>.png

Download Link:
https://<ROOT-URL>/chat/chat/file-upload/8KAZmCZcswoEWQcZt/<FILENAME>.png?download

Relevant nginx conf:

	# rocket chat (docker)
	location ~* "^/[a-z0-9]{40}.(css|js)$" {
		root /opt/rocket/Rocket.Chat/programs/web.browser;
		access_log off;
		expires max;
	}

	location ~ ^/packages {
		root /opt/rocket/Rocket.Chat/programs/web.browser;
		access_log off;
	}

	location ~ ^/(avatar|images|sounds|assets|sockjs|fonts|theme|__meteor__) {
		rewrite ^ /chat$uri;
	}
	
	location /file-upload/ {
           proxy_pass http://127.0.0.1:3000/chat/file-upload/;
        }

	location /chat {
		autoindex on;
		proxy_pass http://127.0.0.1:3000/chat;
		proxy_http_version 1.1;
		proxy_set_header Upgrade $http_upgrade;
		proxy_set_header Connection "upgrade";
		proxy_set_header Host $http_host;
		proxy_set_header X-Real-IP $remote_addr;
		proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
		proxy_set_header X-Forwarded-Proto https;
		proxy_set_header X-Nginx-Proxy true;
		proxy_redirect off;
		client_max_body_size 200M;
	}

Relevant logs:

None

Quick fix / Workaround:

I managed to get this fixed by an additional nginx.conf entry after location /file-upload/ {...}. However, this does not seem to be the preferred way to handle this?!

	# workaround invalid download path v3.12.0
	location /chat/chat/file-upload/ {
           proxy_pass http://127.0.0.1:3000/chat/file-upload/;
        }	

Reproduce the issue using the provided Docker, subdirectory, and nginx configuration, then compare the upload and download URLs shown in the report. Read the related discussion in #20640 and trace the download-path generation from that behavior. Done means downloads work behind nginx in a subdirectory without requiring the workaround location.