It happened with this test spreadsheet. Seems reproducible
TestSpreadsheet.xls (application/vnd.ms-excel), 6.50 KB, created by John Ervin on 2011-01-04 10:06:51 UTC
(hide)
 
Creator: John Ervin
Created: 2011-01-04 10:06:51 UTC
Size: 6.50 KB

Attachment is not viewable in your browser because its MIME type (application/vnd.ms-excel) is not one that your browser is able to display.

Download the attachment.

Actions: View
Attachments on bug 32826: 41632