Trying reverse_proxy X-Accel-Redirect example from docs: unrecognized subdirective file_server

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1

I was experimenting with getting file uploads to work with the soju IRC bouncer and I tried this per the example found in the docs:

reverse_proxy /uploads http://127.0.0.1:3002 {
  @accel header X-Accel-Redirect *
  handle_response @accel {
    root    * /var/www/soju_uploads
    rewrite * {rp.header.X-Accel-Redirect}
    method  * GET
    file_server
}

But I got Error: adapting config using caddyfile: parsing caddyfile tokens for 'reverse_proxy': unrecognized subdirective file_server. Is there a mistake in the docs or am I missing something?

Caddy version 2.10.0 on NixOS.