Download develop

Fix download of images see https://trac.openmicroscopy.org.uk/ome/ticket/10400
To test:
 * Import a single image e.g. jpeg.
   * Download the image
   * Check that the file is the same.
 * Import a multi-image file e.g. lei
   * download the image
   * Check that a zip is created.
   * unzip.
   * Check that all the files are there.