If you have a filename, call Path.GetExtension to get the extension (in the System.IO namespace). Something like:

string ext = Path.GetExtension(fileName);

I would also suggest accessing the filename through the LocationInfo collection, something like:

For Each file As UploadedFile in e.GetUploadedFiles()
    Dim fileName As String = file.LocationInfo(FileUploadStreamProvider.FileNameKey);
    'do something with the filename
Next file