Thank you, Wolfgang. That looks promising. SVG files are converted using MetaPost. I thought the following would work: svg=>{\externalfigure[#1][svg][conversion=mp]} Is there anything else that needs to happen to ensure SVG files are piped through MetaPost? Thanks again!
\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