As pointed out by echicken: when setting the "cost" property to any value, the actual
file's cost would be set to 9223372036854776000 regardless of what number was in the
provided file metadata object. Most jsval numbers aren't doubles (they're ints) so
need to use JS_ValueToNumber() to do the proper conversion, regardless of underlying
type.

Noticed the same problem with the "size" property.

Thanks for the tests and report echicken!