From CONF-17292:

One cause of this problem is using the wrong suffix for Office 2007-style spreadsheets, which are normally saved with a .xlsx suffix. If the file was uploaded using a .xls suffix, Confluence will give it the old Excel spreadsheet MIME type application/vnd.ms-excel.

You can see that this has happened if you use the

parameters to limit the number of rows and columns displayed. Since it bypasses the code that incorrectly interprets arbitrary XML text as row and column counts, it will not cause Confluence to crash, but you will see that all of the cells are filled with XML and Javascript-like code instead of the expected values.

To correctly interpret these files, the file must have the proper MIME-type, which is application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.

Because of the numerous occasions on which these files can be misnamed, and the possibility of crashing an entire wiki, I'd say this is a major problem. At a minimum, making the attachment upload process more persnickety would help a lot.