Just wanted to share some Windows 10 deployment notes from the field. My colleague Ryan Fitzgerald and myself have been troubleshooting file associations not being set properly in Windows 10 build 1511. This has been confirmed from Microsoft Premier Support, that there is a current bug in 1511 that causes DISM not to properly set file associations.
I have tested this on insider build 14271 and file associations are still broken. Insider build 14279 is being downloaded now. I will strip DISM from there, and run DISM 14279 on the 1511 / 10586 upgrade sequence to see if will set the file associations as desired.