I have not gotten any response to this issue yet. Does that mean that everyone else is stumped like I am? Or does that mean that the forum is too busy to respond?
In the meantime, I have run some additional tests. My important discoveries are listed below.
I deployed my ASP.NET app to a Windows 7 machine that runs IIS7. The drag/drop feature of the RadAsyncUpload control works fine in this environment while I am using IE 11.
The difference is this; in my initial post, I was describing my ASP.NET app that is hosted on a Windows Server 2008 R2 Standard edition with Service Pack 1 running IIS 7. It is also a Domain Controller. I am not sure why running the ASP.NET app on Windows 7 vs. 2008 would make a difference, unless it has something to do with tighter security on Windows 2008, especially since it is a DC.
To make this more puzzling, however, I rolled the Windows 2008 server back to IE 10, and it still will not function with the RadAsyncUpload control. This worked before and now it is not working. So now I am thinking that there may have been some additional update, along with the IE 11 upgrade that got installed on this server, which is ultimately causing this problem on the server.
Is anyone aware of any Windows updates that may create problems for the RadAsyncUpload control?