Before this changed we unconditionally fetched the MimeType. On Some
backends like s3 and swift this takes an extra transaction which meant
that `lsf` on those backends was needlessly slow.
This adds an internal option so `lsf` can declare whether it wants
MimeTypes or not depending on whether the user asked for them and an
external flag `--no-mimetype` for `lsjson`.
See: https://forum.rclone.org/t/reliably-setup-incremental-updates/14006/8
bfd9f32188