XLS files with a root node named WORKBOOK don't get parsed

I have an XLS file where the root node contains 2 top-level names - "WORKBOOK" and " SummaryInformation".

The type gets properly detected as "application/vnd.ms-excel", because the POIFSContainerDetector does a check like so:

if (names.contains("Workbook") || names.contains("WORKBOOK")) {
    ...
}

However, the ExcelExtractor silently rejects the file because the root node doesn't contain a top level node named "Workbook".