FIX #527 : Add Thumbnails to 'Choose another file' dialogue

This is part 2 of a fix located at https://github.com/silverstripe/silverstripe-framework/pull/2044

These changes deal with styling the 'Choose another file' modal gridfield of UploadFields to look similar to AssetAdmin.

https://github.com/silverstripe/silverstripe-cms/issues/527