If an image file gets uploaded with uppercase file type extension (.JPG/.PNG) through Connections Desktop Plug-ins preview of images are not available.
The issue was reported for the preview displayed in the activity stream. For example, if an user uploads an image or document, a new entry in the activity stream is displayed including a thumbnail image of the uploaded file. This works fine for files with lowercase file ending (e.g. image.png), but fails for files with uppercase file ending (e.g. image.PNG).