"The thumbnails in the caption page will be displayed only if the file is really ending with the extension jpg.
I haven't been able to reproduce this, .png and .jpg files both show the thumbnail on the caption editing page for me."
Once again, the thumbnails in the caption page don't appear, because it is set generally the file extension ".jpg". Also is displayed the filename always with the filename extension ".jpg, regardless of which it is actually. This is the case on my local system as well as on the test server. Until now, I don't found the cause.