Archiver is missing `Content-Length` header in response

[JammingBen]

Describe the bug

The /archiver endpoint is not returning a Content-Length header in its response. This (possibly) makes it harder for web clients to handle this as a downloadable source.

Steps to reproduce

1. on demo.opencloud.com, select 2 or more files
2. open the browser network tab
3. download the selected files
4. inspect the /archiver response in the browser network tab

Expected behavior

There should be a Content-Length header indicating the size of the archive.

Actual behavior

The header is missing.