In https://github.com/rails/rails/pull/28676 the `#to_path` method was
added to `ActionDispatch::Http::UploadedFile`. This broke usage with
`IO.copy_stream`:

  source = ActionDispatch::Http::UploadedFile.new(...)
  IO.copy_stream(source, destination)
  # ~> TypeError: can't convert ActionDispatch::Http::UploadedFile to IO (ActionDispatch::Http::UploadedFile#to_io gives Tempfile)

Normally `IO.copy_stream` just calls `#read` on the source object.
However, when `#to_path` is defined, `IO.copy_stream` calls `#to_io` in
order to retrieve the raw `File` object. In that case it trips up,
because `ActionDispatch::Http::UploadedFile#to_io` returned a `Tempfile`
object, which is not an `IO` subclass.

We fix this by having `#to_io` return an actual `File` object.