\definefontfeature
  [arabic]
  [mode=node,
   analyze=yes,
   script=arab,
   curs=yes,
   calt=yes,
   ccmp=yes,
   fina=yes,
   init=yes,
   medi=yes]

\starttext

    \showotfcomposition{file:AmiriText-Regular.otf*arabic at 20pt}{-1}{محمد}

    \showotfcomposition{file:AmiriText-Regular.otf*arabic at 20pt}{-1}{لله}

    \showotfcomposition{file:AmiriText-Regular.otf*arabic at 20pt}{-1}{مصر}

\stoptext
