Files in cell output should be discoverable

[kcrisman]

See e.g. http://trac.sagemath.org/ticket/2141, though this has come up before with things like the jmol files.

[gutow]

This is a good point. I hesitate to do anything immediately because I hope to rewrite the JSmol backend, now that I've got the notebook working so that I can serve it behind a proxy again.