In the wiki under \setupexternalfigures one finds for the framing of the figure following in the table: "frames on off"
Mark the 's' at the end of 'frames'
I tested it and found it has to be 'frame' without the 's'.
This is the same misprint as occurred in the earlier ConTeXt manual of 2001.
Perhaps someone knowledageable about changing the wiki can verify this and will be so kind as to update the wiki.