O365 can't open XLT files

HI, i have one user that when she attempts to open XLTX files from Sharepoint online it downloads instead, even if you right click and open in Browser still downloads.

As she doesn't have Office on her home PC this makes it impossible to view file

XLSX files open in the browser no problems as do Word documents, powerpoint etc.

Any help or ideas are welcome and appreciated.

Thanks
Murray

i do not know sharepoint, but it sounds like a settings in the admin panel or server. just a thought.