Thanks for your reply. I was using a handler that I had downloaded from the following site: Use RadFileExplorer with physical and shared folder's paths
After reading your comment I realized that it may be the handler that is causing my issues and I believe I found a solution. I needed to add the following lines of code within the "ProcessRequest" method. After adding these lines I was able to download "xlsx" files and they were not .zip any longer.
Thank you for your help.
Also, when I upgraded my Telerik controls to the new version I did not get any errors. In-fact the analyzer said that the upgrade was successful, and I did not have any errors in my code. At this point, with the older version running, everything is working fine and without errors, but as soon as I upgrade I get the hanging loading panel. I'm hoping that the next upgrade will fix this issue. In the meantime I will use 2015.3.1111.40.