Hello Lin Liu,
Indeed, the version that you use may have such problems. In order to get rid of such outstanding issues I suggest that you upgrade to the AJAX version of the controls.
As for the solution for your problem, you can delete the temp files using the File.Delete method. You need to pass the name of the file (along with the path) that you want to delete. Are the files in the temp folder saved with the same name as in the TargetFolder?
There is another solution for the problem. Don't use RadUploadContext at all. What you can use instead is the .NET Request.Files collection. Here is an example:
/* path */
I hope that this helps.
the Telerik team
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