I have a program generated Planning.xlsx file on my web server. The Mime definition is as follow: .xlsx;application/vnd.openxmlformats-officedocument.spreadsheetml.sheet When the file is downloaded with IE or Safari, the associated program (Scalc.exe) opens the .xlsx file without any problem. With Firefox however, the download program appends a .xls suffix to the file name (Planning.xlsx.xls) and Scalc is unable to read the file whose content does not match the suffix. Changing the default program to Excel works fine but I can't imagine this can be a definitive solution.