Preview of the selected file in the bottom left of the window

[depau]

In the bottom left of the window there's a blank space. That space could be used to display a small preview of the file like in Dolphin, with some details under it. You can recycle the filechooserthumbview's thumbnails to make the process CPU-cheaper (there is a _thumbs DictProperty inside of the chooser with all the previews collected; it doesn't include the picture's thumbnails, however, as they're loaded directly without any process).

By the way, make sure you delete all the files in that dict before exiting, to free the thumbnail cache, as they are not loaded again the next time.