Hi Trimute-Ti,
This is to do with the way the host implements the File dialog. reViSiT does not create the dialog itself. Instead, it sends a request to the host "please show open/save dialog". Therefore, it is up to the people who have written the host as to whether they have included a preview function in the dialog.
However, reViSiT will eventually take control of the open/save sample/instrument/module dialogs, and will boast something similar to the screens of IT - including preview options for sample loading, etc. Though, this is not a top priority right now.
'Hope this helps.
All the best,
Chris