Fix upload of remote media with OpenStack Swift sometimes failing (#16998)

Fix upload of remote media with OpenStack Swift sometimes failing (#16998)

Under certain conditions, files fetched from remotes trigger an error when
being uploaded using OpenStack Swift. This is because in some cases, the
remote server will not return a content-length, so our ResponseWithLimitAdapter
will hold a `nil` value for `#size`, which will lead to an invalid value
for the Content-Length header of the Swift API call.

This commit fixes that by taking the size from the actually-downloaded file
size rather than the upstream-provided Content-Length header value.