Adding extension for attachments

yes, the attachment field is only valid for data objects when those data
objects are children of the template object. it is not valid (and should
not appear) for data objects that are children of item or query objects.
 that's a bummer for parser/validators. not sure there is a way to avoid
this, but it does bug me.​

so we proly need to say something about what to do if/when these show up
inadvertently on data objects when these objects are not children of the
template object. (sigh...)