This parameter replaces the filename and fileobj parameters. This makes
it easier to add more download targets, since only one may be "chosen".
With the OPEN_DOWNLOAD special case added, handling of filename got a
bit ugly, since it may be either None, OPEN_DOWNLOAD or a str with the
file path, and we had to make sure only one target was chosen.
With the new target enum, this handling can be simplified and we
automatically get the guarantee that only one target is chosen.
d42d980dda