Add a default rule to not attempt to cache any files larger than 50MB

This fixes connection failures when trying to retrieve large files.
It is less common in typical Pleroma usage, but it's possible to encounter
this on a cloud instance with lower memory.

installation/pleroma.vcl

@@ -59,6 +59,13 @@ sub vcl_backend_response {
       set beresp.http.CR = beresp.http.content-range;
     }
 
+    # Bypass cache for large files
+    # 50000000 ~ 50MB
+    if (std.integer(beresp.http.content-length, 0) > 50000000) {
+       set beresp.uncacheable = true;
+       return(deliver);
+    }
+
     # Don't cache objects that require authentication
     if (beresp.http.Authorization && !beresp.http.Cache-Control ~ "public") {
       set beresp.uncacheable = true;