[add_header_content_type] Using add_header to set Content-Type

What this check looks for

This plugin looks for configurations that try to set the Content-Type response header using add_header.

Why this is a problem

NGINX can end up sending two Content-Type headers: one from the upstream, and one you added. Different clients handle duplicates differently, and caches may store an unexpected value. If you are trying to set a fallback MIME type for static content, default_type is the right tool.

Bad configuration

# Adds a second Content-Type if the upstream already sets one
add_header Content-Type text/plain;

If your backend returns Content-Type: application/json, the response may contain both headers.

Better configuration

# Sets the default MIME type for responses that do not already have one
default_type text/plain;

default_type applies when there is no explicit content type, so you avoid duplicates.

Safe exception

If you are intentionally replacing the upstream header, hide it first and then add your own:

proxy_hide_header Content-Type;
add_header Content-Type "application/octet-stream";

This pattern removes the upstream Content-Type before adding a new one, so the client sees only a single value.