You could use the build in validation of the RadUpload where you have access to the current RadUploadItem. You can find more information about this approach in this article
However you can not stop file from uploading based on meta-data located in the Request Header. Because once the upload button is pressed and the session is started all files move through the validation which assures the correctness of the files. Also once the session is started you can only pause or cancel the session, you can not modify the Items collection.
I hope this information helps.
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