With .xlsx i cannot open the file.
First message in Excel :
"The file format and extension of XXX don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you wan to open anyway ?"
I click "Yes" and finally :
Excel cannot open ...xlsx because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.
I try to consume the Web Service with SoapUI and no problem to open the Excel Attachment.