Nginx serve another file when possible

Question

I currently compile my assets with Webpack's BrotliPlugin, which creates a separate file, brotli-encoded, i.e.:

• style.css
• style.cssb

Here's what I want to achieve: Consider the HTML page requests style.css.

IF

• The user sent br into its Accept-Encoding request header
• AND the corresponding brotli file exists (style.cssb in this case):

THEN

• Serve the corresponding brotli file content style.cssb
• With a Content-Type: text/css response header
• With a Content-Encoding: br response header

OTHERWISE

• Act as normal, just serve style.css

Here's part of my config so far:

map $http_accept_encoding $accepts_brotli {
    default false;
    "~*br" true;
}
location ~* (.*).(css|js)$ {
    set $brotli_uri "${uri}b";
    set $brotli_file "${request_filename}b";
    set $should_use_brotli "";
    if ($accepts_brotli = true) {
        set $should_use_brotli "Y";
    }
    if (-f $brotli_file) {
        set $should_use_brotli "${should_use_brotli}Y";
    }
    if ($should_use_brotli = YY) {
        add_header X-URI $brotli_uri;
        add_header Content-Encoding br;
    }

    add_header X-FOO "bar";
    try_files $brotli_uri $uri;
}

Result:

• X-URI is present in response
• X-FOO is not
• Uncompressed content is still served

What am I doing wrong?