[NTG-context] Scale image in MetaPost, maintaining aspect ratio

Hi! How to scale an image in MetaPost, maintaining the aspect ratio (without specifying the height explicitly)? Example: \setupexternalfigures [location=default] \starttext \externalfigure [cow][width=5cm] \startMPcode draw externalfigure "cow" scaled 5cm; \stopMPcode \stoptext Marco