Looking for advice!

I created an POI HSSFWorkbook object.
When I'm writing in a file the structure shows as correct.
When I create a file and then subsequently attach to mail the file comes arrives as right.
If I do a direct attachment of the HSSFWorkbook object upon saving,
it turns out to be of the wrong structure and illegible after saving.

parameters : name- "file.xls",
dataSource - HSSFWorkbook.getBytes(),
contentType - "application/zip"

ByteArrayResource res = new ByteArrayResource(dataSource) ;
this.helper.addAttachment( fileName, res, contentType ) ;

what's wrong ?