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.

Fixes #14817

* Change content-type to be always computed from file data

Restore previous behavior, detecting the content-type isn't very
expensive, and some instances may serve files as application/octet-stream
regardless of their true type, making fetching media from them fail, while
it used to work pre-3.2.0.

* Add test