Just to update on this, I also noticed that Windows 10 has a tendency to change the default printer, especially if it's a network/Wi-Fi printer.
For example, my manger has multiple printers set up on his computer - a USB connected inkjet, a networked multifunctional inkjet and a networked laser printer. After upgrading to Windows 10, he ran into problem - His printouts started coming out of his address label printer!
It beats me why Windows decided it would be a good idea to print everything in miniature form on address labels. To save on paper!?
My own computer changed its default to his networked multifunctional.
Another colleague had his default printer set to the XPS document writer, which exports print-outs to XPS files.
As for file extensions, I've lost count the number of times I set VLC as my default player for MP4 video files. From a quick check as I write this post, my PC has already changed the default back to the Windows 'Films & TV' app yet again.