pleroma/docs/administration/storing_remote_media.md

Storing Remote Media

Pleroma does not store remote/federated media by default. The best way to achieve this is to change Nginx to keep its reverse proxy cache forever and to activate the MediaProxyWarmingPolicy MRF policy in Pleroma which will automatically fetch all media through the proxy as soon as the post is received by your instance.

Nginx

proxy_cache_path /long/term/storage/path/pleroma-media-cache levels=1:2 keys_zone=pleroma_media_cache:10m
                 inactive=1y use_temp_path=off;

    location ~ ^/(media|proxy) {
        proxy_cache        pleroma_media_cache;
        slice              1m;
        proxy_cache_key    $host$uri$is_args$args$slice_range;
        proxy_set_header   Range $slice_range;
        proxy_http_version 1.1;
        proxy_cache_valid  206 301 302 304 1h;
        proxy_cache_valid  200 1y;
        proxy_cache_lock   on;
        proxy_cache_use_stale error timeout invalid_header updating;
        proxy_ignore_client_abort on;
        proxy_buffering    on;
        chunked_transfer_encoding on;
        proxy_ignore_headers Cache-Control Expires;
        proxy_hide_header  Cache-Control Expires;
        proxy_pass         http://127.0.0.1:4000;
    }

Pleroma

Add to your prod.secret.exs:

config :pleroma, :instance,
  rewrite_policy: [Pleroma.Web.ActivityPub.MRF.MediaProxyWarmingPolicy]