Adding extension for attachments

again, MUST is a problem in this context. how about:

This extension defines two valid values for the `attachment` property:
 * true (treat this data element as an upload element)
 * false (treat this data element as a text element)
If the client does not support the `attachment` property and/or the value of this property is not understood, the client MUST treat the data element as a text element.

adjust as you see fit.