Prevent changing originalfile.mimetype from 'Directory'

One should not be able to change a `mimetype` in the `originalfile` table from `'Directory'` to some other value. Other changes should still be permitted.

See discussion on https://github.com/openmicroscopy/openmicroscopy/commit/a92d50308b7a82ca9f809e5a3d0d2471bd8b8809.