Hi Jimmy Hill,
Thank you for contacting us.
The Silverlight's OpenFileDialog doesn't support the exclusion mask mechanism. However you can use the FilesSelected event to filter the selected files:
// RadUpload radUpload;
sender, FilesSelectedEventArgs e)
// file.File - System.IO.FileInfo
Please try the suggested approach and let us know if you have other questions.
the Telerik team
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