Firefox showing an error message while downloading .xlsx file

Question

I have uploaded a file say 'filename.xlsx' file to the server. While downloading the file in firefox the file is downloaded as filename.xlsx.xls and i get the error mentioned below while trying to open it. I have no idea why firefox adds an xls extension to the .xlsx file while other browser downloads the file correctly as 'filename.xlsx

The file format and extension of 'filename.xlsx.xls' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want to open it anyway?

Answer 1

Without the link to the file you're downloading, my guess is that the site hosting the file has Content-Type set to application/vnd.ms-excel while it should actually be application/vnd.openxmlformats-officedocument.spreadsheetml.sheet, thus Firefox is giving the file a proper extension based on the information provided by the server.

I've only observed this behavior on Windows. The Linux version of Firefox doesn't add extensions regardless of the MIME type.