Created attachment 5083 File selector: ALL files #WHAT? Thunar does make a difference between File.jpg and File.JPG. One can not upload a file to a web site which allows only files of 'image type'. Consequently XFCE makes it impossible to upload .jpg files, as it does not consider .jpg of 'image type'. Workaround - in case you have understood the problem - is to rename JPG to jpg (convert to lowercase). #WHY? To be done. #HOW? To be done.
This has nothing to do with the filemanager but is an application or toolkit issue.
Thanks for rapid response. If only a toolkit issue, how does this error is only reproducible on XFCE?