Would use Java for ease of cross platform.
File manager can have same theme on Mac and Windows or have a native look and feel on each.
Files will only be view-able, modifiable and delete-able if the logged in user has permissions to do so. Warning messages can be provided if sufficient permissions are not held.
Breadcrumb trail (e.g. C:/ > Users > Admin ....), back and forward buttons, right click menus for copy/paste/delete/open/rename.
Option to view hidden files if needed?