Today's update on Petya
Previously, it was believed that the ransomware would not begin encrypting until an hour after the initial infection. It is now been discovered that it begins encrypting the first 1MB of the below file types upon infection. Therefore turning off your device when viewing the reboot message, will not stop encryption.
It is also now being disputed if the goal of this attack was to collect Bitcoin or cause mass destruction in the devices it infects.