Some linked files open as text files

Hi.
I am linking on my WP sites some MS OFFICE files. The problem is, that while older formats (ppt, xls) work well after clicking the link (the save as dialogue window opens), newer files (pptx, xlsx) open in new tab as txt file (mess of asci characters…). Even if I right click save as on the link, the file saved is in txt format.It happens only in firefox, chrome works fine.

Thanks for help!