At present, it is not possible to configure RadFileExplorer do allow upload of file types different that displayed file types. To achieve the required functionality you can use the following approach:
- Configure RadFIleExplorer to display the required file types - by setting SearchPatterns property
- Handle RadFileExplorer's ItemCommand server-side event and cancel it if the uploaded file is not the correct type.
- On the client handle the upload dialog and manually modify the info panel's text to remove unwanted extensions.
For your convenience I have attached a sample page implementing the above mentioned approach.
the Telerik team