Hi, Sylvain.

LuaMetaTeX and LuaTeX, the engines used by ConTeXt nowadays, use an embedded library, so external compilations/files are not necessary. Just out of curiosity, wouldn't it be easier to use a ConTeXt environment? E.g.:

\starttext

\startMPcode %\startuseMPgraphic{<name>} if you want to reuse your graphic with \useMPgraphic{<name>}

    draw fullcircle scaled cm

    %withcolor black % black is default
    ;

\stopMPcode %\stopuseMPgraphic

\stoptext

More info here: https://wiki.contextgarden.net/MetaFun_-_MetaPost_in_ConTeXt
To actually answer your question, if you actually want to load a Metapost graphic externally generated, you should have something like:

%nice.mp

%"begin ... endfig", as well as "end" are important

beginfig (1);
draw fullcircle scaled cm
%withcolor black
;
endfig;
end

so when you compile it via

mpost nice.mp

(obviously supposing you have Metapost installed) you'll get a file called nice.1 and

\starttext

\externalfigure[nice.1][width=4cm]

\stoptext

will work. However, as you can see, ConTeXt deals with such minutiae for you and extends Metapost capabilities too, so the second alternative is not the ConTeXt way to go.

I hope it helps.

Jairo :)