David:
There is another approach possible for this that I have discussed in the past. See http://wwrk.maa.org/moodle/mod/forum/discuss.php?d=1950 and https://devel.webwork.rochester.edu:8002/webwork2_files/moodle/mod/forum/discuss.php?d=2775 (see my response hear the bottom).
These point you to a macro file that I wrote that allows you to set up a problem that gives the student a choice of several related files that they can choose from. It is not exactly what you are looking for, but could be modified to work more like what you want.
Davide
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