With reference to this forum thread
setting the extension in the choose file dialog is not technically possible. RadUpload has been replaced by RadAsyncUpload. The official support for RadUpload has been discontinued in June 2013 (Q2’13), although it is still be available in the suite. We deeply believe that RadAsyncUpload can better serve your upload needs and we kindly ask you to transition to it to make sure you take advantage of its support and the new features we constantly add to it. So one suggestion is that you can use RadAsyncUpload to Upload files. Please check out this help documentation
for more information. Let me know if you have any concern.