We have already answered to you in the support thread. I will share the answer here, for community reference.
Unfortunately RadUpload (as well as ASP.NET Upload) can't remember its value across the postback. This is actually a limitation of the browsers, which do not allow one to set the value of the <input type='file' /> element using scripts. Since the latter element is used to upload the files to the server, the uploaded files will be lost after a postback occur. The only thing that you can do in order to workaround this problem is to validate the rest of the inputs on the client, so that no postback is needed. Every postback will upload the files selected in the file upload, no matter whether it was a validation postback or a file upload postback.
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