Download zip

Fix the remaining problem when downloading the zip directly from the activity window. (see https://trac.openmicroscopy.org.uk/ome/ticket/9961)

To test:
 * Create a zip using the ```Batch Export``` script (select one image).
 * Download the result from the activity window.
 * Create a zip using the ```Batch Export``` script (select 2 images).
 * Download the result from the activity window. A dialog should pop up to indicate that a file with the same name already exists.