I found a funny video on youtube, but I am not totally convinced by the argument. It says that a pure magnetic field caused by a current in a wire can lead to a combined electric + magnetic field in a moving frame. It explains more or less convincingly that this can be understood as a consequence of length contraction observed in the moving frame. The contraction would break the neutrality of the wire. I do not understand why the electronic current would not be contracted in the rest frame and why it would not also lead to an electric field. At time 1:35, the speaker says that positive and negative charges would be the same. Why?