You can't because, due to sandbox security, you don't have access to the filepath!
You can simulate this by immediately upload the image to a temp directory on your server and then link the preview image to this temp path. When the user clicks ok, you can write a script on your server to move the temp file to it's final location.
I seen examples of an image getting loaded into a canvas.
What if I used this as an Item Render field in the grid?
If you can get the filePath, it's easy to use that in your dataGrid. Put the filePath in a property of your dataProvider and then make a simple itemRenderer to display it. In this itemRenderer, put an image with it's source property bound to the filePath property of your provider: