\setupexternalfigures [location={local,global,default}]

\defineexternalfigure [jpg] [width=4cm]
\defineexternalfigure [png] [width=8cm]

\starttexdefinition includegraphics #1

    \splitfilename{#1}

    \processaction
        [\splitofftype]
        [    jpg=>{\externalfigure[#1][jpg]},
             png=>{\externalfigure[#1][png]},
         default=>{\externalfigure[#1]},
         unknown=>{\externalfigure[#1]}]

\stoptexdefinition

\starttext

\includegraphics{hacker.jpg}

\includegraphics{mill.png}

\includegraphics{cow.pdf}

\stoptext