\externalfigure doesn't find mps file in subdirectory

[Sanjoy Mahajan <sanjoy@olin.edu>]

\externalfigure won't find a metapost eps file in a subdirectory, if the
mps has the numerical (.1) extension.

Here's a minimal example (test.tex):

\setupexternalfigures[directory={fig}]
\starttext
\externalfigure[simple.1]
\stoptext

where fig/simple.1 is from a simple .mp file that fills a 1-cm box (see
below).  

The resulting test.pdf shows the gray box 

  name : simple.1
  file : simple.1
  state: unknown

Only this combination of circumstances triggers the problem.  Giving the
full path ("\externalfigure[fig/simple.1]") works fine, as does renaming
the file to simple.mps (within the fig/ directory), as does using
instead simple-1.pdf (made with mptopdf).

Thus, I have several workarounds, but it might be useful to figure out
what is not working.  I've noticed this problem on and off for a while
now, but only now made a minimal example.  (Is the path-searching code
trying to append extensions and directory prefixes at the same time, and
thus not finding the file?)

This is with MkIV 2013.05.28 from vanilla TL2013.

For completeness, here's simple.1, although any file will show the
problem equally well:

%!PS
%%BoundingBox: 0 0 29 29 
%%HiResBoundingBox: 0 0 28.34645 28.34645 
%%Creator: MetaPost 1.803
%%CreationDate: 2013.08.04:0841
%%Pages: 1
%%BeginProlog
%%EndProlog
%%Page: 1 1
 0 0 0 setrgbcolor
newpath 0 0 moveto
28.34645 0 lineto
28.34645 28.34645 lineto
0 28.34645 lineto
 closepath fill
showpage
%%EOF


-Sanjoy
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