File associations are also stored in the registry under HKEY_LOCAL_MACHINE\SOFTWARE\Classes and HKEY_CURRENT_USER\SOFTWARE\Classes. You can change the default association here if needed. If it appears that the functionality to associate a default app is broken in Windows 10 Technical Preview, please submit the issue via the built in Feedback app to notify the development teams.
You can also look at defaults through the new PC Settings app. If you are still unable to assign the default you desire, you can provide us with more details, such as the extension you are trying to associate and the application you are trying to associate it with. A screenshot would also help in attempting to reproduce the issue.