Using an AC plug for DC current
If it's rated at 10A you can't go higher than that safely. The amp rating should be based upon the amount that the contact resistance will cause the plug to heat up. If the current is higher than this, regardless of the voltage being supplied, the contacts may start heating up too much causing issues. 

Generally speaking for plugs, the voltage and amp ratings are for different things. The voltage rating is for preventing shorting from happening do to dielectric breakdown. The current is for determining how much heat can be generated within the device.

If it was me, and I wanted to be safe, I would stay under 10A even at 12V DC.