Best content-type and extension for web delivered Excel 2003 SpreadsheetML file -

I am working with an API that has a report URL call. The report has the "Excel 2003 XML" format option. When it is chosen, then they distribute it like this:

  Content type: application / vnd.ms-excel content-disposition: inline; File name = "report-name.xls"  

3.0 OpenOffice.org tries to bring it as text import and I am worried that Office 2007 users expose mismatch can give . This should not be our problem but people like to shoot Messenger, so I am looking for suggestions about file developers about researching file developers and suggesting.

I open .xlsx or .xml and OO happily open the file, but the file is not actually a .xlsx file, .xml is annoyingly normal and I do not know that office 2007 What will you think about

What are the best content types and extensions for the Web to avoid these and other issues? Excel 2003 (11) Spreadsheet files to distribute?