I don't know about implementation of this because last time I tried Nautilus, it
had the same annoying problem. But essentially, if you have a file that shows up
with a thumbnail in thunar, that thumbnail will never be refreshed. Common use
cases where you would want it refreshed are:

1. You have file1.png and file2.png. If you delete file1.png and rename
file2.png to file1.png, it will still show the original thumbnail.

2. If you have a file1.png, edit it in gimp, save it, the original thumbnail is
still shown.

It's very hard to work with images when thumbnails do not properly refresh.

Reproducible: Always
Steps to Reproduce:
1. Have two image files.
2. Delete the first image file, rename the second image file to the name of the
first.

Actual Results:  
The thumbnail of the first image file is still shown.

Expected Results:  
Updated/refreshed the thumbnail.

I'm using thunar-svn 19738.