This was fixed a few days ago, here’s the related issue: https://github.com/solid/node-solid-server/issues/1361
This isn’t the error cause for the solid-filemanager though. I guess the cause here is, that it sends
content-type: undefined while it (probably) should guess and specify it. I’d refactor it sometime after the solid-file-client update. If you want it to be changed earlier, PRs (which don’t break other stuff obviously) are accepted.