From the provided information it is hard to determine what was causing your issue. It could be an issue with the Flash module that is used in case no Silverlight is installed (IFrame is used if you do not have neither silverlight nor Flash installed in IE). Did setting the DisablePlugins
helped resolve the issue? Please note also that if you are using the IFrame the maximum file size that you can upload is 4MB, thus, if you are using the IFrame and try to upload a larger file you might experience issue.
As a side note, the reason for the RadAsyncUpload to work properly in FireFox is that you are using a different module in the respective browser that is not causing issues.
the Telerik team
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