I was looking also at this image from Wikipedia, which shows decoupling/bypassing as something any type can be used for:
Then if that is what it shows quite simply it is wrong and needs correcting.
Have you read this:- http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html it tells you about self resonance, the point where a capacitor stops looking like a capacitor and starts looking like an inductor and so stops filtering out the interference.
I still have not much of an idea really what I should be filtering exactly - what frequency/voltage/etc
You are doing two things:-
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You are lowering the source impedance of the voltage input to the regulator. The motors will have the effect of raising this source impedance.
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You are trying to remove very short spikes of under and over voltage caused by the motor. Because they are short then they are a very high frequency.