This issue is observed only in R1 2020. The solution is to not disable the FileAPI.
Reproduction steps:
<script>
Telerik.Web.UI.RadAsyncUpload.Modules.FileApi.isAvailable = function () { return false; };
Telerik.Web.UI.RadAsyncUpload.Modules.Flash.isAvailable = function () { return false; };
Telerik.Web.UI.RadAsyncUpload.Modules.Silverlight.isAvailable = function () { return false; };
</script>
<telerik:RadAsyncUpload runat="server" ID="RadAsyncUpload1" ></telerik:RadAsyncUpload>